Shouldn't a '42-'50 truck light duty 3 spd just bolt in, the 1/2 bell on the
engine should match and the clutch setup should be the same. I know some of the larger trucks had a different cross member under the trans, but assuming you have a 1/2 ton based on your F150 designation. You will need a different drive shaft since the light duty has a splined output and the 4spd has the saddle output. That will not give you the overdrive though and from what I have seen has pretty weak gears (I have yet to find one with good gears)
There are a lot of people swapping in the 80s S10 T5 transmission, search here for that swap. Your setup should take the after market
parts to do this swap also. It is also possible that the trans side of the 1/2 bell on your setup will unbolt and can be used as an adapter for swapping in a newer style transmission.