I have a 7AC-CONV with an O-235 and it is a good performer. I fly it on PK1500 floats in the summer and 26" AkBushwheels when off floats. I would think a much heavier GCBC might be a little "sluggish".....just my guess though. I would want an O-320 in a GCBC. I think the usefull load in the GCBC is pretty marginal also. A friend has one and I believe my useful load on the Champ is more than his GCBC.....on floats anyways.
WW