Hi Russ.
Lot's of good info being provided to you. I would encourage you to keep it simple for your first time using an AN/ARC-5 transmitter. A variac on your HP power supply to give you 550V and 3 resistors (20K 5W, 15K 10W, and 100K 1W) will give you the 3 voltages you need. The voltage divider is what you'd find in the BC-456 modulator and I assume the MD-7 as well; it's how the system was originally designed and it's worked well for 85 years.....
I've attached two documents from Dave AB5S which are great for someone starting out with AN/ARC-5 or SCR-274N radios. You'll find the design for the voltage divider in one of the documents. Dirt simple and it works.
The transformer in the HP-23 looks to be stout enough so you should not see much sag on key down. If you do, and the resulting chirp is too bad, then regulation for the oscillator and screen may be necessary.
73 Mark K3MSB