Notice the 51C has cathode series feedback from the OPT secondary. This is the same as the Leslie 145. There is quite a bit of similarity between these two schem's and I suspect that the OPT is the same as the regular 122 or 145 style Leslie amp.
In the 122, which has no secondary feedback I have replaced these 6550 tubes with EL34s with a very minor modification (see below). I suppose it could be done to the 145 configured amp. The modification gives slightly less power but a much better overdrive characteristic. Overdrive is blasphemy for some organ players. They like it clean, clean, clean. OTOH, some like a bit of grind. For these players the EL34 gives a nice easily controllable overdrive. IMO the stock 6550s sound like crap when overdriven, giving a gravely crackly noise that corresponds to the periodic signal peaks induced by the Hammond's chorus/vibrato.