Good point!
I am pretty much at this bridge just now with a Model 80 upgrade / restore mod, luckily though... there are molex connectors with the 80.
Regards to your system, and if you are intent on using a standard SCSI drive with molex connection... a DIY mod sounds like the way to go. Can you solder?
If so it should be an easy mod to make... just locate the trunk PSU cable that goes to the motherboard. Open the PSU (not as daunting as it sounds) and find the source of the trunk cable. Patch (solder) a single cable coming from the 5v DC line, one from the 12v DC, and two from common (ground). Thread all 4 of them through into the chassis and attach a connector. This as someone suggested earlier would be easier if you just butcher a set of cables with a connector already on it from another PSU and thread it through from the chassis into the PSU.
Once you've done this, you can get splitters and have many 4-pin molex connections by using a splitter from the single one you've made.
All the best!