An Amphenol type plug might be the best option. We use them at work. They aren't cheap. The type 67Dart273 shows we term "clickie-lock". They are the latest/greatest. I recall buying a few 53-pin ones some years ago for ~$100 ea. You would do better to search for surplus ones on ebay, especially for the older 1/4-turn or multi-turn ones. I have had clue-less managers order us to dump boxes of new Amphenols still in the package in the dumpster during clean-ups for CEO visits. We did so, but went out later and recovered them ($20K worth). I am sure some government facilities sell them cheap as surplus on the pallet, thus they show up on ebay.
Another question is why you need a bulkhead connector. I expect the factory used them to speed assembly. Most cars no longer used them after they changed to fuel injection and the number of wires went way up. I know when I have stripped 80-90's GM trucks (for HEI & such), the harness runs straight thru the firewall, with just one big grommet.
