I still have 4 Mustangs. The 2017 'Vert has about 10K miles on it now and the wife wants to keep that one for road trips. The '84 T-Roof will probably go up for sale this spring. The '64-1/2 is still sitting in the CONEX where it has been for the past 20 years. So, that leaves me the '65 coupe...
Ford still makes all 3 roller cams that were so popular back in the 5.0's days of glory.
My own 5.0 has an Isky roller cam that's a little hotter than the stock 5.0 HO camshaft that has a ton of torque.
Ford made a transmission in 77-78 that externally looks just like a top-loader but is actually a 3 speed with its 4th gear being an OD. I wonder if that would work? If so, here are some tag numbers:
RUG BP
RUG CA
RUG CD
RUG CL
The only 1966 Mustang blue interior paint was Medium Blue Poly.
Beyond that you will need to figure out if the car had standard buckets, standard bench, or the interior decor (pony) buckets.
After that you can figure out what the trim scheme was.