B/C Body 4 speed into an A-body.

Ok I did this swap
I mean I put a long shaft tranny into my 68 Barracuda.
Daliant got everything right.
I had to grind off most of the top forward-most mounting lug, just for clearance to the top brace, in the tunnel That whole entire area would have to be reworked to even bolt on a plate.
Fortunately my extension came with the rear mount as well, and I used it to put the shifter up very high,and in just in front of the buckets. Awesome!!
So the answer is; the long shaft tranny is awesome in an A, waaay better than the other, and I highly recommend it. But,you HAVE to have the rear mounting pad, and I don't think you can ever get the shifter into the stock location. Close is as good as it gets.
And you would have to build your own shift-rods which is a good thing, cuz the stock rods are way too flexible. I used some seamless tubing that was sized to accept the cut off ends, and plug-welded them in.I raised the shifter high enough that the top mounting bolt is above the floor.My stick is only about 8 to 10 inches long.