I put a Top Loader (sweetest trany ever!) in my old 67 Tang w/ C4.
It was easy. The brake bracket is the same, the trany mount is the same the cross member too, even the drive shaft was the same, i was glad the spine matched. the speed-o gear is not.
I had to use a chain and turnbuckle to hold the left side of the engine down because it wuld rip the engine mount apart regularlly. it was bolted to an rippin 289, the SVO aluminium roller rockers made a huge difference. Huge!
this was in 1985. i got the fresh toploader for $800 back then, i dont what they are today.
Alternativelly, a used t-5 is goog and plentyfull, easy to rebuild too. rebuild kit from Hanlon Motorsport is under 300.
Dont know about the mounting but i have some old magazine articles on that swap that i can scan.