Check and see if you have to use a 65-66 trans mount. My crossmember is the modern driveline unit, made for the swap from an auto to a t5. On their site it says the 65 auto. trans mount will bolt up to the t5 then to the crossmember. I didn't read that part of the site and tried to use the 1991 trans mount that was on my t5 when I got it. I went ahead and drilled a hole in the crossmember and I swear literally 2 minutes after my buddy came outside yelling "don't drill it!!!" Whoops, too late.
He had checked the site while I was drilling (d'oh!)
Then I looked at the mount that was on my c4, and it was in two pieces. Great. So I had to order one and it was a full week before I could drive my 5 speed. All because I didn't read. By the way, the hole made no structural difference, but I wish I didn't do it.