Anyone swap a Tremec T45 in their 67-69 Dart?

I have a 1969 auto dart and I want to swap it to a 4-Speed or 5-Speed. I found a tremec for a 99-04 mustang and wondered if anyone knows the pain-in-the-*** it might be?? I was going to do the 3-Speed OD swap but also like the idea of the Tremec but they are $5,000! Anyone know the headache a T45 would cause me?? Haha.

1969 Dart @ Bobs.jpg