Boy, that's sad.
Can't argue with your experiences, Xpolsiv65. My 250 was rebuilt before I got it, and has been so reliable, aside from some 'backyard butcher' doing the head gasket in.
The 3.8 'Essex' V6 is an optimised, built for no rod offset block. It mayt have a 90 degree angle and the same mount positions, but it has an offset phasing between each banck of cylinders. On the good old Windsor, the cylinders are split one half cylinder. On the V6, it is some thing like t=one half cylinder, plus 100 thou for the 30 degree offset crank pins. That's why the block on this engine is so amazingly light. There is no fat at all. The steering would be nice and light with a such a light engine.
The block isn't bullet proof, but is good enough for most people. Just remember, Noise Vibration and Harshness is worse in a 65 Mustang because the spring tower braces and firewall transmit vibration back to the driver. It's not really an issue with an in-line six or V8, but a V6 may be as noisey in your 65 as an early Pinto four or Mustang II 2.3.
Then theres the fuel tank return line, electric fuel pump, crash cut-out, EFI module and fitting the sump and drivers side header past the steering box. Do you want Maverick power steering or just run a 16:1 GTspec steering box. These engines have aready to run weight of around 310 pounds. The AOD adds over 100 pounds on a C4, while a T5, if you can get one for it, is about the same weight as a C4. The 200 weighes in at around 385 pounds, ready to run.