Have you checked and replaced the PRAM battery? That's the first thing I'd look at doing.

I had a similar 'type' problem when recently trying to resurrect a IIfx. The hard disk wasn't spinning up even though I knew the HD was good. I tested the PSU voltages and they were fine.

On inspecting the logic board I noticed a shorted circuit with a break in it, a drop of solder later and a reassemble and the HD spun up and booted fine.

If your swopped in PSU is known to be good and the swopped in HD is also known to be good, and you've replaced the PRAM battery with a new one - I'd be looking at the logic board for possible circuit breaks.

The fact the machine boots from floppy but there's not enough juice to spin up the HD does rather point to the PRAM battery for me.