When I bought my '63 Effie ICB, it had a '64 390 PI engine swapped in. It was done by using a pair of aftermarket 'TRANS-DAPT' crossmembers. Both had ends that simply fit to, and bolted onto the frame.
The first time I pulled the engine, I unbolted the engine crossmember and removed it with the engine. The rubber 390 'isolator' engine mounts have a stud that sticks down at an angle and wouldn't clear the slots in the crossmember pads. I took a cutting torch and extended the slots upward before I re-installed the engine, and from then on it wasn't necessary to remove the crossmember.
I used several fender washers at each end of the trans crossmember to adjust for proper engine/trans angle to the rear end.
The guy I bought the truck from had installed it using '63-'64 427 long exhaust manifolds. This made it necessary to modify the factory crossmember at the firewall that takes the rear ends of the parallel front leaf springs. He notched the crossmember underside considerably and fishplated the bottom to make holes the exhaust pipes could run through. Otherwise, the exhaust manifolds dumped into the forward face of the crossmember with about three inches clearance.
Those are the 'catch-points' to the installation. From the hood emblem, my truck was also a '6' from the factory---with an automatic. The 'z'-bar wasn't in perfect alignment when I got it and I re-worked it a bit for use with the Top-loader and used Heim-joints and steel Hurst shifter bushings to take the slop out of the clutch linkage. I also used a Lakewood scattershield.
KS