There are 2 different 6X heads. The 101cc 400 6X has a a secondary stamp '8', the 94cc 350 6X head has a '4'.
You can see the '8' or '4' stamped on the side of the projection next to the valve cover rail on the sparkplug side of the head. The 350 6X heads would be ideal, with a little milling you would have a nice 8.5:1 compression engine that is pump-gas friendly.
The 400 6X heads can be milled to get compression in the low 8:1 range. You may get a bit more compression if your engine has been bored oversize .030" or .060".
If you keep the head milling to .030" or less you will have no troubles fitting the intake, otherwise you'll have to mill the intake side of the head to compensate.
Yes, the '75-1/2 -'77 6X heads will bolt onto the '65 389 block (or any other Pontiac block). The '65 Tri-Power intake will bolt up as well. You will have to fill the 'dead hole' above the exhaust crossover on the right head with steel filler epoxy and use the '65 -'71 intake gaskets to seal the crossover.
Here is some info on crossover matching by Jim Hand.