Well, for starters you would need to change pistons in the 7M block to match your 5M head since the 7M pistons have valve reliefs cut for the 24-valve head. The engine mount location on the 7M block for the MKIII would also have to be changed to allow you drop the 7M bottom end into a MKII. This is no big deal since the MKII engine mount locations exist on the 7M block. Mike Lewitz has done this hybrid build. Maybe he could help you out and give you a few more pointers. OTOH, why not just do the popular 7MGTE conversion and be done with it? There are plenty of MKII owners running 7MGTE conversions without any complaints and are quite happy with the results.