RE: big block in a 10th scale redcat racing tornado buggy.
I have only ever done one big block conversion and it was several years ago on a T-Maxx. I ran into basically the same issue as you did. I ended up drilling new holes in the chassis and running some shimms on the motor mounts to get my desired hieght. I ran it for a few year like that without any issue.
I wish I had a better answer for you, and maybe someone does here. Good luck.