The 3.8L and the 4.2L are both 90 degree V blocks with the same deck height and bore spacing. This means that even though the heads and intake manifolds are different they will interchange. The following info is a copy from a previous post I responded to that might be of some interest.

Don't know if anyone is still watching this string but there is something all 4.2L V6 owners should now. The 4.2L is a bored and stroked brother to the 3.8L found in Windstars, Mustangs and the old Thunderbird Supercoupe. I have researched this because I was trying to make a 4.2L supercharged engine out of 3.8L parts and stick it in a Mustang.

This project never evolved for me but I discovered that the cam, upper intake, supercharger and heads from a 3.8L Supercoupe engine will fit on a 4.2L engine. Most of these parts are still available over the dealer's parts counter but I would find a total Supercoupe T-Bird if possible. I never researched this swap for a F-150 but the first real issues you will come across is getting the FEAD system to operate correctly. The FEAD system on the 4.2L must extend about 7 inches forward of the engine block and the 3.8L Supercoupe's FEAD is very tight and close to the block so this will be an issues to over come. FEAD = Front End Accessory Drive