I done it with /6 harness and cut/splice/solder/heatshrink the connection for a clean looking install.

done a few mid 80s trucks swapping in /6-SB-BB engines.

I have unwrapped an 85 harness and removed all but a few needed wires to run my 88 440 truck bypassing the stock IGN. and BR along with any emissions crap under hood.

on a 70s it should be 3 plug at the firewall correct?

remove/unwrap it and remove what you don't want/need and cut/splice/solder/heatshrink it up with new wire.

I have cut up a bunch of truck wire harness so I have a box of correct color/long pieces to patch with.

I would look at a /6 harness as it will have most of the connections toward the front like a BB

or find another harness to cut up for patches for a clean up what you got now.

should be ez pz to mod it to work.

I swapped 3 harness out in my 79 RC/JY truck in about an hour time. removed 2 full 3 piece harness and installed 1.