I used a pair of Hammond 1650G OPTs in a triode connected ST70 clone and it sounded great. However, since I don't believe in chassis hacking, I did have to make a set of aluminum adapter plates to mount the trannies in a stock ST70 chassis. If money is an issue I would buy another 1645 and spend a little of the extra money in beefing up the power supply capacitance.
If cost is not an issue and you want to build an true ST70 clone then use Ned's trannies. I have a set in one of my amps and they look and sound great.
FWIW, I built an ST70 for a friend and used a power supply from Cyrus Chong,
cyruschong@yahoo.com. It is much improved from the original ST70 power supply and mounts under one of the OPTs where the bias circuit usually goes. It supplies a lot more filtering and has a zener diode regulated bias supply. I was much impressed. The only problem was what to do with the hole for the cap can...lol
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.