Exhaust--with a twist

Disclaimer: I have a '93.
However, many of the specs for exhaust will translate, and due to the slight age difference of the trucks, this operation might have become necessary for yours before mine. Looking for veterans of exhaust system upgrades who can offer wisdom.

So then, here's what I am thinking:
My current exhaust system is almost worn out; cracked-up manifolds, missing tailpipes from the muffler back, and I suspect the dip in fuel efficiency has to do with the cat being 25 years old. Almost upgrade/fix time, and I lean heavily on the upgrade side.
There are currently no known problems with the EGR system, so I would like to keep it intact, I think (feel free to change my mind).

What I think I know about what works best for exhaust for the 302/5.0:
1. Long tube headers improve low end torque.
2. A crossover helps flow.
3. In order to keep the ship's computer happy, a cat (or two, depending?) is needed between the O2 sensors.

Also, for frivolity, I have been imagining a double tailpipe on the passenger side in front of the rear wheel.

With that as my direction so far, I open the door to anyone who has expertise or strong opinions backed by solid scientific theory.

You know that I don't have any of the qualifications you requested.  But, you also know that I have strong opinions - about almost everything.  

First, you need to check that long-tubes will fit.  And by "fit" I mean not only will they clear the tranny and other such obstructions, but that they won't be against the frame.  Big Blue's L&L's, which are about the most expensive header you can buy, are against the frame on the driver's side.  And I've seen someone comment on FB that their L&L's are against the frame, but on the passenger's side.

So for that reason I'm not sold on long-tubes.  And since these won't fit the 460 EFI heads I'm considering shorties for the up-coming changes.  In theory shorties flow better and bolt into the same spot as the manifolds.

I, too, like the dual idea, so will go that way as well.  And, I like Magnaflow muffs, so will certainly go that way.  But Magnaflow makes a dual-in/dual-out muffler that might work if two singles won't fit.

Yes, I think you need cats or the 'puter won't like it.  And EGR doesn't hurt the performance, can help the MPG's, and certainly helps the environment so is good to retain.

I'm running a set of BBK shorties on my 302...

https://www.summitracing.com/int/parts/bbk-3510/overview/

I don't know how much extra low end torque you would gain with long tube headers over shorties.

The only long tubes I've seen listed for the 5.0 trucks say that are specifically for the auto trans trucks only. I'm not sure why that is....but I assume it is because of the hydraulic clutch line (or clutch linkage) is in the way on the LH side? I dunno.

I went with the shorties so that I could run my duals in basically the same location as the stock Y-Pipe and single exhaust. It worked out perfectly in the end. I have full duals with an H-Pipe, and no cats (not required for me area).

The computer does not have a problem with no cat(s) unless you have the OBD-II required post cat O2 sensor. I am running dual O2 sensors on Darth, one in each down pipe right below the manifolds. The post-cat O2 sensor is turned off in the software along with the misfire detector so they will not set a code. Darth has the late 1984-1987 dual exhaust pipes to the muffler then the single tailpipe over the axle and out. I am not running headers due to previous experience with them on two different cars.

FoxFord33 wrote

...I suspect the dip in fuel efficiency has to do with the cat being 25 years old.

No, cats don't age.  Mine are 23 years old with nearly a million miles, and still work like-new.  If yours are damaged, it's due to the exhaust leaks UPstream of the oxygen sensor (HEGO) causing the EEC to think the engine is running lean, forcing it to actually run rich, which burns out the cats.  Click this & read the caption:

FoxFord33 wrote

...I lean heavily on the upgrade side.

The best, quickest, cheapest, & most-effective upgrade is MAINTENANCE.  It's an upgrade because so many vehicles are neglected for so much of their lives.  If something is damaged or ruined, try a top-quality direct-replacement part before looking for something that claims to be "better" in some way.

FoxFord33 wrote

1. Long tube headers improve low end torque.

Maybe.  But they certainly create frame, starter, wiring, & emissions system problems.  And they usually leak if they're not re-tightened frequently (which NO ONE does).  And they can melt nearby plastic, and cause the A/C to not perform as well.  And they co$t more.

So their many drawbacks FAR outweigh their one (dubious) advantage.

FoxFord33 wrote

2. A crossover helps flow.

Compared to what?  The factory exhaust already crosses over, so another crossover can't help flow any more.

FoxFord33 wrote

3. In order to keep the ship's computer happy, a cat (or two, depending?) is needed between the O2 sensors.

A '93 only has 1 HEGO (O2), and it's before the cats.

FoxFord33 wrote

...a double tailpipe on the passenger side in front of the rear wheel.

That'll only add weight & cost - not performance, durability, or longevity.  And if the engine isn't running right, it'll make a mess on the wheel & the paint.  The factory outlet location is the best place.

FoxFord33 wrote

...anyone who has expertise or strong opinions backed by solid scientific theory.

I stayed at a Holiday Inn Express a few years ago.  Is that close enough?

I ran Hedman headers on my 351W, and assembled the rest with pieces of 2 1/2" tubing with clamps, using some Jegs mufflers as it was REALLY loud without them - almost NASCAR loud :)

Headers: https://www.summitracing.com/parts/hed-89270

... into two collector extensions that looked like this (can't find the exact model):

Collectors: https://www.hedman.com/product-detail/18700

I ran two separate pipes, and had to run both in between the cabin and transmission crossmember, and then dumped the driver side downward, just before the first gas tank - and did the same with the passenger side.

Pics for reference:









I don't have pics of the end pipes, but they terminate just after the mufflers at about a 90 degree downturn (straight down, not angled out the side).

Hope this helps.