Lots of good info here.
The 8.5" will be plenty for 500 hp. If you're going to be shocking it a lot, I might consider a cast/preloader cover.
When shopping, don't use the bolts on the bolt cover to determine what the axle is. 10-bolt axles are named 10-bolt because of the number of bolts holding the ring gear to the carrier, not the bolts on the cover. It is true that most 10-bolt axles have ten bolts on the cover, but not always. There were 12 bolt axles with 10 on the cover. It's not always correlative.
The 12 bolt will be a nice step up. 14 bolt axles came in two different forms; a 9.5" ring semi-floater and a 10.5" ring full floater. Both are overkill and I don't think any of the 14s came with 5-lug. Several came with 6 lug. Since the 14s were 3/4 ton and up, the only factory ratios you'll find are a bit lower (higher numerically), but if you dig hard enough you can find 3.23 and 3.42.
All of GM's axles were available with the G80 RPO. G80 does not indicate any specific TYPE of traction device, it just simply means that it has something other than an open diff. Lighter duty trucks often got a friction-type posi; either a Auburn cone style or an Eaton flat clutch style. As you got into the heavier trucks it was usually the one they called the gov-lock. This one functioned as an open diff until a wee little spring sensed wheel spin and engaged limited slip clutches. This is the one that got a bad rep, but it is completely fine and actually a great diff. It got its bad reputation from drag racing. People would put slicks on a camaro with 600 hp and take it to the strip. Since it is normally open, they would launch, it would light up the left tire and get it spinning at 50 mph, then a split second later it locked up the other slick and things went boom. In a street truck, it is a wonderful diff. I have one in a 10.5" FF that came out of a 500k-mile P-step van and when I took it apart it looked brand new.
The important take-away there is that G80 means some kind of posi, but you won't know which one until you open it up. Much like RPO LT1 means something totally different in 1970 than it does in 1995... or 2019. I wish GM would stop recycling RPOs. They used different posi types over the years and applications based on newer technology, who was the lowest bidder, what the market could support with cost vs. sticker prices, etc.
I would suggest a truck 12-bolt. They don't carry the same price tag as car 12 bolts because they are perceived as weaker... which they are a tiny bit, but only because of the pinion bearing being more similar to a 10-bolt in the truck 12s. It's a bolt-in, it was available with 5-lug, and the aftermarket has you covered with anything.
But, if you find an 8.5" 10-bolt, I wouldn't pass on it either.
Double check that it is, in fact an 8.5". The factory never put the wimpy 7.5" in trucks (except for a few years of trying them in the C10 stepside) but there was a TSB on some 8.5" axles back in the 80s, and GM's response was to replace the axle... which was sometimes a 7.5". They're pretty easy to spot from the outside. 8.5s have a mostly round pattern to the cover while 7.5s are wider than they are tall. A 7.5" rear with 500 hp will last up until your first time going full throttle. Then you'll have to push the truck backwards to collect the carrier which has been ejected through the cover.
good luck on this journey.
In reply to Curtis :
