I put a 1975 350 with HEI in my 1964.
Seems I recall getting a set of brackets from a doner truck that had a V8 in it and they were different than the straight 6 that was in there. There are 3 holes along each frame rail and I mounted my brackets in the two forward holes. This was done to give me clearance between the HEI and the firewall.
I put a 700R4 behind my engine. You will have to remove the 3 speed cross member (riveted on) and replace it with a new cross member. I got mine again from a doner truck only this time it was a 67 to 71.
And as stated you will have to shorten the drive shaft (can't remember if the yoke was different due to the splines).