Krullibulls Ae86 BEAMS is the only way

[Ivan141]

If your line of work allows access to new parts then that changes the story somewhat.
I've done options 1 and 2 once in the past, but without access to all the parts and tools
just splicing wires was much easier and more reliable.

From a quality standpoint it makes more sense to use all new wires and pins. From a practicality standpoint it makes more sense to leave the engine loom as is and splice it to your ECU loom at a convenient location. If you do it the way you mentioned with crimps it should be just as reliable as OEM (they also use crimps as you surely know). The only downside being that you need to spread out the splices to avoid the loom becoming too thick in one point.

About bulkhead connectors: Have a look around. If it's good enough for most motorsports applications, then surely there must be a solution that works.