The reason newer ATX power supplies have less capacity on the 5V line is that CPU power has long since moved to using voltage regulation from a 12V source. Back in the 68k days, CPUs were still being run directly off the 5V rail.
Okay, I've done more research on PSUs.
The original Quadra PSU outputs the following voltages:
+5V Trickle 'Constinuous' @ 1.25A
+5V 'Normal' @ 33A
-12V @ 1A
+12V (a) @ 5A
+12V (b) @ 5A
Modern PSUs can output most of the voltages above with a higher amperage, except for the 5V rail. This is most likely due to the fact that the standard molex connector has been phased out. The more molex connectors a PSU has, the more powerful the 5V rail tends to be. Your average brand new ATX PSU can provide up to 20A on the 5V rail, same thing with the industrial ones. To put things into perspective, even the AX1600i, Corsair's flagship 1600W PSU can't go past 30A. Geekdot's PSU must have been a couple years old already as it too can do 30A.
I've checked with Seasonic and most of the other well known brands, and I couldn't find anything relatively new that does more than 28A.
I think I'll go with the Corsair VX550, a 40 euro PSU on the second hand market that can do 5V @28A. It also has 6 molex connectors. I'll only need four.
For the adapter board, I'd like it to handle a couple of things:
1/ Standard Molex to P4/64 Bit Power 4 Pin Square Molex conversion (DC side)
2/ Soft power (DC side)
3/ Monitor out power (AC 'hot' side)
I'll start working on it right away.