I did some research on this, as I was planning to do the same thing, but decided against another stock frame build. But I was going to put in just the lower rear motor mount and the leave the upper open like Walsh, laeger and some lsr and arens frames. I was going to build a 3/16 or 1/4 lower rear mount, beef up the lower middle one, then tun stock headstay and front mounts.
2 things to watch when doing this, swingarm/case clearance, and don't twist the engine.