I haven't got a clue what i'm doing...

I'm thinking of getting an overdrive to stick behind the ford crossflow 250ci inline 6 and 3spd manual 'box i'll probably use in my mini rat wrecker project. I'm thinking of getting an f-350 dually diff (wheel to wheel to wheel to wheel complete rear assembly haha) which is probably going to be from a diesel truck. Will it even need an OD to cruise at 100km/h at low rpm if it has the tall ratio of the diesel diff in it?

What old OD assemblies are out there that are easy to adapt to various boxes, easy to use (easy on other vehicle mechanicals, girlfriend friendly), and easy to find/lots of them around/plenty of parts for repair etc etc?

You should be able to adapt a T5. I am doing one on a 260 that was a 3 speed and the 2 bottom holes already line up. Since the T5 has a longer shaft, the thickness of the plate won't be a problem. I am using 3/4 inch aluminum. I had my machinist cut the register hole.
diesel 70 rears are about the same ratios as gas. I have a mid 80s 6.9 F350 dually rear and it came with 3.73 but the owner switched to 4.10

they must change the ratios in the gearbox then. or the diesels pull higher revs than i assumed?

If I remember correct, 1st gear was different on the diesels, but 4th was the same.