Starter Problems Please Help

[G3500]

Hello, I have recently replaced an engine in a 97 G3500 Van with a 5.7. We replaced the engine with a 5.7 from a 99 Suburban. Now no matter how many shims or bolts the starter jumps, it may crank good for a minute but eventually jumps and grinds. The flexplate is on correct and no teeth are broken yet the starter is a new one from pepboys, what on earth are we doing wrong. We have been shiming across the whole starter and have not tried one side only, At this point me and my father in law are confused.

 

thanks

 

 

I Have tried a starter from a suburban and it did not engage the flexplate correctly. My father in law says it almost looks like the flexplate is flexing, there was no starter brace on the old engine and the flexplate cover is off.