Definitely need a different trans, even more-so if you have the SM465. The overdrive 833 wouldn't really be any better, our R&D guy at work has one in his '67 Satellite with a mild 360 in front of it and it's a dog. A T5 would be ok if you get a V8 or World Class one and don't drive it like a maniac. I've had 3 Mustangs with the T5 or T45 version and beat the snot out of them, they were fine. A truck is heavier, that's what would eventually hurt a T5.
I'd say an NV3500 or an AR5 would be a better budget choice. Or if you can swing it, one of the new Tremec TKX's would be killer. For a gear swap, keep in mind your tire size too, a 3.42 would be great but if you plan to do 20s with a taller tire, I'd step up to a 3.73.