ok, for the first part of your question. A 351c will only fit in a 64-66 with sever massaging of the shock towers. It should fit just fine in a 67-68. And as for puting a T-5 in a early mustang, go to
www.windsor-fox.com they specialize in restomoding. And they will have all the info and parts you will need to make that tranny work in the car.