Swapping heads will be twice the work. not only do you still have to set up the EFI, you also have to pull the head get some machine work done to it and put it on the 4.2. With the higher compression of the 4.2 you'd also have to run higher octane gas for it to run right. If you want it running well and soon just put the 4.0 in.

The fuel pump needs to able to supply a constant 40 PSI. You could either buy a 4.0 tank and pump, or put in an aftermarket pump.