Missed 1 item - for EFI you must run a late model alternator - it comes with the 4200 so its a no brainer.
Would there be any interest in a u-fab engine mount kit with the engine plates ready to bolt on tubing and ears to fit over a '70-'80's frame cushion (engine mount)? Cut/weld the tubing to the plates & ears.
With the wiring and PCM out of the way the hardest part is running the fuel lines.
51 GMC 4.2 turbo
Can't solved today's problems using the same technology/thinking that created them