Bulkhead wire size

Skip Year None, Van's Auto and others sell M&H.
Do a search there's a TON of info on wiring. I made dash harness for our 67 cuda.
Looking to redo my engine harness for my 66 bcuda either from scratch myself or with a year one m&h harness with a few mods to not overheat and burn up the bulkhead connector. Any tips, tricks, sources for engine compartment connectors etc?

All input appreciated