I have a 1991 FLH bottom end. Can I replace flywheels with Shovelhead and install Shovel top end ?
I've never done it, I know it can be done, but I wouldn't. The reason is the cases have to be welded up. filling the cylinder stud mounts. Then the case needs to be drilled and tapped for the Shovel cylinder bolt pattern, they are different than Evo.
I did do a Shovel lower and put an Evo top and flywheel in it. Same thing had to happen, welding and tapping. In the end, new cases would have been a better solution. The motor went into an Evo FXWG frame.
I'll add that eventually the cylinder studs would pull and cause base gasket and head gasket leaks. So we used new S&S cases a few years later.