Fair enough. Rebuild the underhood wiring harness *first* with all modern wires and connectors and you’ll avoid a lot of nightmares before, during, and after the swap. Also, make sure the TH400 behind the engine is properly rebuilt. Next, unless it has the later GM-sourced HEI ignition system, convert it to the optical or CDI aftermarket ignition controls. Then install an aftermarket transmission cooler and a larger (+1 core) radiator of good quality to avoid the thermal kills so common to Brit cars of the era. Finally, fit the car with at LEAST a 105 amp (bigger is better) alternator and upgrade the battery and alternator leads to avoid a lot of the electrical problems with the car. You may also wish to fabricate some electric fans for it.
rolls has a gm tranny? cool