Is the manifold flange size on older (ok..old) Gil exhaust with 3" pipes the same on the newer Gils with 4" pipes? In other words, can I put newer 4" pipes on the older manifolds?

I don't recall Gil ever making a 3" exhaust. I worked for them for 8 years during the 90's and all Big Block sytems were either 4" or 4-1/2" OD tailpipes. They did do a small block system in the late 80's and early 90's that used a 3-1/2" tailpipe. That manifold was discontinued about 1993. After that all small block systems went to 4" OD tailpipes.

They discontinued all small block systems earlier this year.

The bolt pattern of the tailpipes they build today is on a 4-1/4" square so just measure the center to center distance of your manifold studs.

If you need tailpipes made for the older style 3-1/2" small block manifolds give me a call, I may be able to find some of the old flanges to build a set of pipes for you.