Auto trans. to manual questions

[airdale94]

Hi, I have a 68 C20. Originally a 6 cyl. manual trans., PO swapped a 283 TH350. I'm swapping back to a SM465, bought everything from a donor truck. I also bought a complete front "stub" from a 84 and swapped all that except for the crossmember. They were exactly the same except for the rubber brake hose mounting points.
I want to upgrade to newer style (clamshell) motor mounts. It looks like they will be in a middle position, as opposed to a forward (V-8) or rear (6 cyl.) position. Will I have to drill some new holes in the frame for my trans. crossmember? I'm thinking I will. How hard is this to do? Anyone have any experience with this? Should I just use the original motor mounts in the forward position for sb V-8? If I do, shouldn't the trans. crossmember bolt in the original location?