Yes, you can use a 91-93 5.0 wiring harness and computer to run a 351W, however 94 has a different dash, with a different dash harness that will not directly mate with the 91-93 engine harness. Also if you do that, you will be converting from OBD2 to OBD1, so while that is your best option for a 351W swap, it is not a plug and play swap. As for the 351W blocks, 94 is when they went to the roller cam block. I have heard rumors that they were still using a flat tappet cam in the roller blocks for a while, but I have never seen it. It isn't really relevant though, since you would be changing the cam if you went that far anyway. Also they do make small base circle cams to be able to run a roller cam in a non-roller block. For the trans behind a 351W swap, you have a few options. For a manual trans, the full setup out of a 5-speed SC will bolt right in. For an automatic, you can use an AOD (or DOA as I like to call them) out of any 89-93 MN12, which will be a direct bolt-in swap, or when you do the wiring, you can add some wiring and run a 94/95 auto Mustang GT ECM, which has the provision for transmission controls and will allow you to run a 4R70 out of a 99-03ish V6 Mustang, which is a way better trans than the AOD, and at that point you could use a driveshaft out of a 94-97 V6 MN12. The only thing that might require a custom driveshaft would be a C4 or C6 swap, or a T56 or something like that.
As for the 05+ TR3650 swap, that definitely will require a custom driveshaft, since it uses a shaft with a flange at both ends and the slip yoke built into the driveshaft. That adds to the expense of the swap, and you will also need to get a speedcal to get your speedo right with that swap as well, since the TR3650 uses a magnetic pickup for the speed sensor, which has a drastically different number of pulses per revolution than the old speedo gear style. The nice thing about that swap though, is that the factory hydraulic throwout bearing works with the SC master cylinder, and the shifter comes up right in the correct location, so there is no real fabricating required, and it all pretty much bolts right in.
On the paint, that last pic with the hood really shows the metallic in that blue, and I bet the car looks better in person than in the pics. Only thing I don't like is it makes me cringe every time I see one of these cars painted with the quarter glass still in place, and with all the stuff you took off, I'm curious why you didn't pull the trunk lock cylinder too?
-91 Cougar LS, coming soon, complete overhaul
with a 427" Windsor.
-90SC 5-speed, MP2, coated rotors, double intercooler, 15%OD, ported heads, comp stage 1 cam, 85mm TB, 90MM LMAF, 80# injectors, soon to be transplanted into...
-90 XR7 5-speed black on black w/sunroof
-98 Mark VIII LSC, Procharger P600b, TR3650 swap and 3.73s.
-70 Torino GT Fastback project car. Needs EFI and a manual trans, but I'm not sure what motor to go with yet.
-"Your buddy Mike is INSANE!" -ClintD's dad