Well then it pretty much comes down to how much he wants to spend, ya know? If he wants an increase in power yet is on a fairly tight budget then get a set of GT40 cast iron heads and work them over good. A GT40 intake (upper and lower). Stock 5.0 mustang cam or GEN 1 Lightning cam.
If money isn't a problem get a set of AFR 165s. The GT40 intake and extrude hone the lower or a TFS or Edlebrock. Custom cam ground for torque.
Or middle budget of combining the mods above. The internals will be fine going back with hypereutectics like the stock ones. I don't know what the stock compression is on that motor but you could also raise the compression some more in your combo and get some more N/A power since it's not going to have forced induction.
What tranny is in it? If it hasn't been rebuilt yet then I"d say that'd be in order to do, too. Since I see you mentioned "convertor" is it the AOD or E4OD? It's probably the E4OD and I'd get it rebuilt if it's got that many miles on it (though I don't see it even making it that long, lol). You should talk to Gregg Evans of Factory Tech Transmission. He's a former Ford tranny tech and really knows his stuff.
Here's a link to his forum over on the National Lightning Owners Club website.
As for the rear end throw in a set of 4.10s for sure.
Ah, and don't forget the exhaust. Especially for torque look into a set of Hedman or Hooker long tubes, with custom catalytic midpipes and your choice of catback (Magnaflow is really good).