generally, the backplate being on wrong causes the crank to stop on the down stroke, it actually makes the piston skirt hit the backplate, that is why some are machined for piston skirt clearence. If you remove the head and back plate, rotate the assembly and there is no lock up, then install the back plate and try it again, if it still rotates fine, then your problem is lack of head shims or the rod is the wrong one. The only thing you changed was the rod and as long as everything else was put back together the way it came apart, then the rod is wrong. I would almost bet the design changes between the g-75 and sk-75 included a different rod, this is more than likly a wrong part issue than a internal issue else where. If you want to verify it, remove the new rod and line them up one on top of the other, they should be identical except for the defect in the old rod, any slight difference resulting in a longer rod will be an issue.