I just registered and saw your old post.  Probably by now you have your problem fixed...anyway I grew up on a 48 oliver 70.  My dad in the 60's had oversized pistons and rings put in.  He said it gave the same power as a  77.  You may find it increases your 77's power, if it is possible, as well.  Also, I remember that the carb could sometimes be finicky and get out of adjustment or the float would stick.  I would first check to see if it's running too lean.  If that doesn't help, before you go into anything expensive like a rebuilt engine, you might rebuild the carburator.