Graupner speed 280`s will quite happily run on 7.2v, or at least they do on my converted Severn, and a pair of non-flying boats, and a plastic battleship and and…
They seem to work well enough on 25mm brass props that I have fitted to them. The Severn is particularly…brisk. One I have seen with two 400s was even brisker, if a bit lower in the water, but the attitude at speed was well improved by some transom flaps. You may not want these as it will make the boat a bit less original, but thats not much of a price to pay so you can enjoy THRASHING all the other tubs around the pond. Sorry about that it just slipped out.
You would want to be embarrassed by not enough power, eh?
They have the advantage of being much smaller than the 400`s needless to say which may be a plus point if the boat is non too big (should have googled to see what size we are talking about here first), and should think they would provide about as much power as a 400, practically speaking, in a non-planing hull..
Ashley