Is there a difference in C10 & C20 motor mount frame perches?

[Mathew21]

I have a 1978 P10 step van. I'm swapping out the 292 for a 6.0L LS style motor. I had an extra set of C10 motor mount perches that I'm trying to use. When I try to line up the bottom of the perches with the holes on the crossmember the perch has a gap on top of the frame rail. I thought I read somewhere or someone told me before that I would need to use C20 perches. I was wondering if they are shorter than the C10 ones.

Thanks for any help