Hello, I'm trying to purchase a 3.36 or 3.23 ring and pinion (haven't decided yet) for my 68 and I was told they are hard to find because it's simply not a camaro. I'm confused because I've found some from various sources so I'm not sure if I'm missing something. Should I worry about buying from different distributors or from sellers like eBay selling a used set. Just wanna make sure I'm buying the right ring and pinion for my application how do I confirm this. Thanks.

First look to see what you have now. There are several different carriers, a 3.23 or 3.36 will need the carrier of a similar ratio. Example: if you have 2.56 now your new gears won't fit on that carrier.
Check the letter code on the left axle tube to see what you have now.

Assuming your rearend is factory, they have to be compatible with an 8.2" BOP rearend, which is different than a Chevy.

Check the letter codes and then check the actual ratio by putting on a lift and turning the drive shaft and wheels/tires. Also, count the number of bolts on the differential cover, if original, it should be a 10 bolt. Finally, compare what you have to the pictures/references at jdrace.com

As far as purchasing parts, I've gotten both my rearend gears and posi units from Randy's Ring and Pinion. I think they go by the name Randy's Worldwide now. It isn't the cheapest option but I've had no issues getting the right parts.

Thanks for the responses. It's 10 bolt stamped zp so I think that's 3.9?

Which is a special rear end in that someone purposely ordered it that way. It was standard on Ram Air cars and available as a special order ratio on certain 350 HO and 400 & 400 HO cars. If you haven't already, I would order the PHS documents to be sure what you've got.

I checked Rocky Rotella's recent book but didn't find any rear gear breakouts.

Your ZP is a posi rear end that originally came with a 3.90 gear ratio (39/10) gears. I just went through a gear change myself and found that it is near impossible to find a gear set you can use in your carrier. They splines you need are BOP specific and most pinion gears do not have the 27 spline. If you change carrier and get a complete set of carrier and matching gears can be done, but if you're particular about retaining quality and originality you may decide against it. Plus if you research carrier manufacturers you soon find out they have noisy and unreliable units. Best bet is to talk to a reputable gear shop, in the area and pick their brain. I ended up to leaving the carrier stock and added a shim to be able to use a higher gear set using a stock gear set. Worked out great, you can do it yourself, take your time, get a mag base indicator set, shim stock and slowly do it in 2 weekends. I have 2 OEM sets out of my Firebirds up for sale. One is a 2.78 the other 3.55 set if you're interested.

Ramair68 from what did you change your gear ratio to? After hearing your experience/insight keeping the 3.90 might be an option now. I have two rear ends one with 2.78 and the original with 3.90.

What transmission are you using?

I went to 3.08, TH400. IMO 3.90 unless you have a manual tranny is good for the track and burnouts, for a good all around and still be able to take er up to 120mph I would recommend to stay at 3.55 or a little below. There is a table out there that calculates the RPM in any speed with your gear ratio installed. That's what I did when I picked mine. You have to tailor to your driving habits and desired outcome before you pick. With my 3.08 I can take my car up to 160mph and still be a tat under where redline starts, but if you never go above 100mph the 3.90 is good, but you would redline probably at around 120mph with an AUTO Tranny. And unless you like high pitch whining noise, lower gear rations are better on the ears. Last but not least, think about what you're gonna do with your engine.....it all affects gear ratio, everything has to work together....

Use this: https://spicerparts.com/calculators/engine-rpm-calculator

TH400

I don't know much about the carriers. So would these not work?? It says it is for 3.23 - 3.55 but 28 spline. Is ours 27?

Yukon POBCarrier

The 28 refers to the axle splines, those are 28 and 27 spline to the driveshaft yoke. This is what I used, much better, keep your original carrier....https://www.amazon.com/Bolt-Vette-Ring-Spacer-Bolts/dp/B00SDI83Y6.

Did you try rockauto they have the Dana ring and pinion and an other brand for the bop rear end and don’t change the carrier you can get the correct spacer you will probably have to get longer bolts for the spacer the ones they come with are not long enough
They have them in stock I just looked 390 373 355 336 273
You have to watch their are carrier breaks for the different gears so the spacer will be a different thickness anywhere from a 1/4 to 1/2 inch
Theirs a chart on this somewhere
Make sure you look at 8.2 bop

Also FYI, with a 3.55 rear gear and my original TH 400, I am turning 3,000 rpm at freeway speeds (65 mph)

That is some decision making criteria I did not consider in my rear gear ratio selection.

As mentioned above, cam, convertor, tranny, rear tire diameter,... all factor into the decision. if you're changing your carrier, why not go posi-traction? This allows you to wear out both rear tires evenly.