Jess, if you have the original eng and trans, you will have vibriation issues. It's the way the engines were balanced. must2 used a reduced- diameter c4 bellhousing, torque converter and a 141-tooth flexplate among other parts. I just rebuilt mine, so I've been there. I may still have the part number that I used if you need it. my email is bullitt1@charter.net.
I'm no expert but I've ran in to a lot of issues building my 78 mach 1
Larry
As far as changing the balance on the flex plate, any reputable automotive machine shop should be able to help you out. But Larry has pointed out that the II does use a specific flex plate as well as a specific bell housing. So, if you change flex plates, you will need to change bell housings. Just something to keep in mind as you continue your build.