If you are dead-set on keeping the v6, it’s possible to get mid 12 second quarter miles out of it. With a 4door explorer, that number is probably more around mid 13's. It’s totally possible, and probably cheaper then swapping to a stoker v8. 4k will get you there. 10:1 compression, overlap cam, ported heads/intake, 75 shot of dry nitrous. The 4.0 handles up to 300hp reliably, and can be ran completely on the stock computer.
Although getting the right tranny combo is going to be interesting. If you don’t have a 5-speed stock, you probably want to look into a C4/C5 hybrid, or a 700r4 conversion before going to any serious power out of the 4.0
I've heard some talk on the RPS forums that the M5 is practically identical to the T5 internally. I don’t know how legit that statement is, but it sounds good to me.