All the radio cares about is the proper voltage and current in the right places, not the pedigree of the power supply. Be aware the Swan 400 has approximately twice the transmitter power as similar radios of that vintage which means you cannot easily modify a Heath
kit PS-23 or similar. They're not heavy enough to supply the Swan at full output. Your next best bet would be to scrounge a nasty old tube style PA amplifier with a truly large power transformer, but most 6L6 type amplifiers ran 450 to 500 volts on the plate. The Swan might want nearly twice that voltage. Buying a new transformer is possible but not inexpensive, see http://hammondmfg.ca
It also looks like the Swan 400 used sweep tubes in the final: http://www.insites.us/halo05d.html
This is not a desirable characteristic. The published specs say transmitter distortion products are suppressed by at least 30 dB. By modern standards that's not a good number and the FCC would have issues with the radio if it was being sold today as a current model.
You can find the documentation at BAMA: http://bama.edebris.com/manuals/swan/400/
If you want to drop a Franklin as tuition for a practical learning experience that's OK, but not every old transceiver is worth restoring and there's a good chance the radio needs more than just a power supply... It's pushing 50 years old and there will be issues. My inner cynic says there's a reason why the current owner didn't replace the power supply but moved on to newer and better gear instead. Might have decided the radio has the potential of becoming a money pit and could be correct in that. You might be better off waiting until you learn more about which used radios are worth looking at then put the money toward something that is both complete and has more potential than a Swan. I have a feeling the Swan will be available for a while and there's no rush to buy it now.
There will always be a bigger & better deal in the future........