Ive got a mid 70s 400 going in a 68 bird and there is maybe an 1/8 inch clearance from bottom of pulley to front xmember. Did the first generation 400 take a smaller pulley than the later models.

I was working on my car tonight and looked. I could put my index finger tight between pulley and cross member.

Billy check to see how much clearance you have between your oil pan and crossmember. You may be in need of some new motor mounts or some spacers.
There were different sizes of crank and wp pulleys depending on application like AC or other factors.

The motor mounts are brand new. The car is currently at the resto shop and the owner contacted me yesterday and told me of the issue. He suggested raising the engine up about 1/2 inch or so. Someone else suggested an underdrive pulley which is about 1 smaller in diameter than my current one. That seems like a good route to go but what are the cons of going with a smaller crank pulley?

Is it an 8" A/C pulley or 7.75" non A/C pulley?
Can hardly get a finger between pulley and frame on 68 non a/c setup.

Its the 8 inch for ac