Bass-ackward amp selection...

Just wondering if there are any Pass DIY designs that can make use of a dual mono power supply with +/- 75 volts? I have a huge PA amp that died leaving behind a carcass and a big power supply. I plan to gut it and put something DIY in there. Unfortunately my skill level is modest, so "tweaking" an existing design is not a good option. Any suggestions?
+-75Vdc should be good for 200W into 8ohms.
Two channels from your power supply will require a big, well designed PSU.
If you use 4ohm loading the power demand is doubled. Can your PSU supply that doubled demand?

For big power amps, you will find that MJ21193/4 are excellent in the output stage. 1pair could supply 16ohm, 2pair for 8ohm and 4pair for 4ohm would be a good starting guess.
Pass are ClassA designs.
They are generally not suitable for enormous ClassA power into low impedance loads.

His extremely expensive get up to around the target power you have in mind but they cannot be powered by a skimpy PSU from a PA amplifier.

You need to re-assess your requirements and then see if the PSU suits your new target.
Any specific Pass designs I should be looking at? The original amp put out about 450 watts into 4 ohm.
in terms of what you stated here, any PA PSU will be skimpy.

Was that specification 450+450W into 4ohms?
If so then that would indicate a transformer VA of between 900VA and 2kVA.
It should also have between 20mF and 100mF on each supply rail and these should be rated for 100V duty.