I don't know the t3/t4 flows, but you should be able to find them on the web.

The 3.8 will have a block deck 19mm higher than the 3.0, just like the 3.5. All other dimensions should be the same.

You will need to figure out a spark system under MS, probably a ford edis variant using the mitsu trigger wheels, or use an sohc with a dizzy. I'd make the effort to get the 24valves.

Re the radiator, when you raise the hp, you also raise the waste heat, and thus the size of the radiator needed. You also need to allow for the water and oil cooling of the turbo and those heat loads.

Both the gen1 and gen2 radiators are roughly the same size, as is the starquest turbo radiator. The starquest has a 2row, and the 2.6 carby truck has a 3row, and the third row raises the heat rating (approx 177500btu/hr) by almost exactly 50%. Early 3.0 radiators were also 3row, but I believe they went to 2 row in 90.

That rating for a 3row means you could waste the equivalent of almost 70hp to the water jacket, and you won't do near that.