Is it possible to put a 4.6 mustang motor into a 1994 ford ranger.?
You're going to be in for some fun. You've should have just enough room in there to fit (i'm assuming SOHC as 26 inches) , but I believe you'll have to cut back the steering column and depending on how you've got it set up may need to change out the exhaust manifolds (though it sounds like you're going with headers, so those should squeeze things in a bit assuming it's fairly short, probably better of going with something from the mid-70's).
You'll find you're using mostly parts from your donor car, I don't think you'll need to do completely custom parts, but you may find some strange bedfellows in your parts, I put a SB 350 from the 70's into a 57 Chevy at one point and found I was using all sorts of parts, from 4 decades and 3 manufacturers.
Lastly, expect this to take you somewhere between 50-75 hours if you know what you're doing. If this is your first engine swap be prepared to double that, so realize it's not a one-weekend project.
Forgot one thing - Different engine means different stress points as well - you'll want to read up on that and make sure you get good engine mounts, with the extra power expect to replace the drive train, or at the very least the transfer case (probably going to do with a dana 20?)