What is the bolt pattern measurement for a 69?

5 on 4-3/4"

Thanks!

Next question is that will a Chebby pattern in that size fit and what is the largest tire that will fit? I have found a set of chebby Camaro wheels from an 2012 that are 18" witha 4 3/4" pattern

I used this website to lookup compatible wheels:

http://www.roadkillcustoms.com/hot-rods-rat-rods/Wheel-Bolt-Pattern-Cross-Reference-Database.asp

What I found was that every Camaro had the same bolt pattern as a firebird, there were other GM cars also but were mostly more obscure. I was interested in the newer Camaro/Firebird compact spares. I think you may have a hard time fitting 18" in without modification. I have some fat 15" and they tend to rub a bit.

Remember to check the center bore to be sure it's big enough to fit your Firebird.
Can't help you with what the biggest tire would be. Been talked about many times here though.

Chevy S10 wheels have a 5 X 4-3/4 pattern as well. My grandson has 235/70R15 on his and they look good.