Ona C Body, Not so. Worst case is a few small bends in the floor. I've done 2 516 conversions. The world famous Polaraco, which was the first 518 conversion done in the world according to Rick Ehrenberg. He had dozens of B Body guys looking for help. Sent to me. The hardest part is the trans mount. It has to be sturdy as the T Bars pivot on the member. The other is a 62 New Yorker. Again, a few small dents. Mount was a lot easier. I have developed an electric shift which works great! 6 years! You can use the original push buttons too. Right now, I have a problem with RF coming from the fuel injection. Working that out. I won't release it until I have that worked out.I suspect you might have more issues with the 518 clearing the existing trans hump than getting it to accept the pushbutton cable mechanism? Reason I say that is that I read, a while back, that some modification/enlargement to the trans hump on B-bodies were needed. Might be similar on the '64s?
What rear axle ratio in the '64? Multiply it by .7 (as a general rule) to get the resulting "in OD" effective axle ratio. Might need a rear axle ratio change to a deeper ratio so the OD can be used at normal highway speeds, FWIW.
CBODY67
I'm selling the design so I won't be producing it. I'm too old now. Like Stan. LOL