Best brand ring and pinion set.

Have an 8.8 3.55 on my 85 f150 302 efi 4x4 AOD. Motor has slight mod to around 300hp. Fist time I've ever rebuilt one. (I Have some help)What do I need to know about kits and any upgrades?

I used a Yukon set from RA on my best friend's 10.25" Sterling. Never did an 8.8", but have done some other similar ones (GM and MGB Spicer style).

I re-did my 96 mustangs 8.8
I went with 4.10 from ford performance. And I've heard that Yukons are good too. If you are going to replace the gears. Do a complete rebuild. There are numerous complete kits available. Bearings, seals, shims and even gear fluid with a separate bottle of friction modifier.if it comes with the cover gasket throw it away and just use a high quality RTV. When replacing rear end gears, you need to break them in according to the manufactures recommendation. Also I had a differential specialist shop do the work. $335 with a 1 year warranty. The stock 8.8 in our trucks should have no problem handling 300 hp. I've seen some guys use the 8.8 with just axle upgrades i.e. 31 or 35 spline. With motors putting out 800 hp. The 8.8 has plenty of gear and axle upgrades, and even brake options out there at reasonable prices. Not to hijack your thread, but I wonder if the Mustangs rear disk brake setup would work. I've seen the conversation kits that use GM parts.  
I converted my stock GT rear brakes to 11.5 Cobra. All it required was the cobra mounting brackets. But I'm sure stock 15" rims won't work. When I did my cobra conversion I had to go up to 18" rims. Again sorry for the hijack,

Then they got bought out, and expanded their line substantially.

You'd need to investigate recent customer satisfaction. I almost used them a couple years ago, but found out my metal in oil problem was a failed wheel bearing, not the R&P. They were polite and helpful on the phone, I must say.

How do I find the spindle count? Other than taking it apart. Was looking at the kits.

I have Yukon gears in the 8.8 of my 2003 Ranger, and they've been great for the last six years I've had them in there.