Sure it can be done, but be forewarned, it's a fairly involved project that will probably cost more and take more time than you think. Especially if you're going with an EFI 302 or 351. The 351 can be done, but it will take more custom work since it is taller and wider than the 302.
The AOD automatic transmission is the most common swap transmission used, but you can also use the T-5 or actually the Chevy 700R4. Advance Adapters makes adapters to mate these to the 302.
If you use a carbuerated motor, it will make everything simpler, but hey, it's 2001, who wants to use a carbuerator.
In my opinion, the hard part is all the little stuff, wiring, brackets, radiator, new accessories, bolts, etc.
Where you get your motor and transmission makes a big difference on your cost. If you buy a "take-out" salvage motor and transmission, you can save a lot of money. But then you would be installing a used motor. How lucky do you feel? You could rebuild both, and the price starts going up. You can buy brand new everything, and believe me, that's not cheap by any stretch. But the reliability definitely goes up.
If you're serious about doing this, you need to start doing a lot of research, unless your custom fab guy is going to take care of everything. If you just have to pay him to take care of everything, that would be easy for you. But again, to swap an EFI motor and transmission into an Explorer is not simple by any means.
