I have a 3.55 in the 67 with a Th400 and love it.

IMO, depends on how hot the engine is going to be built. If you have a low performance, small cammed 350 with 2 bbl heads, you wont have use for anything steeper than a 3.08 or 3.23. You wont have the torque.

Now if you replicated the 350 HO with #18 or #48 heads with 65cc's and an 067 or 068 regrind and decent compression and headers or longbranch manifolds, I would go into the 3.36 or 3.42.

The 350 has the same stroke as a 400 and likes to be wound out as quick as possible. If you don't have the grunt in the low end, doesn't matter what gear you have, it will fall flat on it's face.

I had a 400 with a Torker II intake, 3.55 gears, 9-1 compression with a 245/ .510 Lunati single profile and a 3500 stall. It had nothing off the line in first and shifted at 4500. Once I got the gear up to 3500 RPM it would freight train in 2/3. I changed the heads to #13's and the [censored] thing screamed through all gears and had a traction problems even with 275/60/15 rears. The engine got even better changing to a stock intake. So..I guess my point is...You need compression and good flowing heads for a decent gear to work under any circumstance.