Someone's gonna blow something up! =) Good luck =-\ and wear saftey glasses, in the case of that many caps at 350 volts body armor might be a good idea too. I've dead shorted one of those before and they give a pretty healthy bang, 50 is easily lethal.
The flash circuit should be able to charge the bank just fine, it'll just take about 50 times longer. If you want a decent simple schematic how about the one you already have? You should be able to reverse engineer the flash circuit you have, finding a bigger transformer might be a problem though.
Do a google search of this website and look for high voltage supply you should find plenty of examples floating around here, this isn't the first time someone's wanted to do this.
If I'm not mistaken you should be able to use a few diodes and capacitors to voltage double and rectify 120 volt mains power, but you'll want to do some kind of current limiting, you're definitely into dangerous voltage and energy potentials.