Hi Gerben!
I would suspect that your issue might be more related to a very high output rear end and slow gearbox rather than the engine?
If it can rev OK then the speed limitation is otherwhere in the drivetrain.
I believe the gmc straight sixes were governed up to 3500 RPM, but i'm not sure.
If you are going to change your engine i would be interested in buying your other 302, i'm located in Sweden.
Take care!
