This with a small center section from a donor 93/53 and your in business since the cuts are straight. This used to be done all the time before the time of flats with a simple jig and those are more readily available now that we have flat bending and rail profile jigs to modify where before they were not and you had to make them. This even give someone the ability to add the correct HK serial etc and with laser engraving now thats a ton easier.
But now with the sp5 in play, the only thing your really buying that is not readily available is the mag well sling loop. Granted the front half is welded together correctly/done and most likely in spec and THAT is say 90% of the build and complications that guys here are worried about.
However you MUST be careful to add a filler pin and a SA shelf that covers the pin and make it non-removable first before reweld or viola you have created an unregistered machine gun.