Tuned port is a HUGE step backwards compared to vortec technology. Vortec technology is the mid 90's lt1 cylinder head technology, but adapted to standard coolant flow instead of reverse coolant flow like the lt1 was. 350 and 454 use completely different wiring harness's. Small blocks and v6's used spider injection where 454 used multi port. Throttle bodies were mounted completely different, and the 454 used the large long runner intake compared to the small blocks compact cross runner with plastic upper. If you pick up a 454 with harness, I'm sure you could do an 0411 pcm swap and wire it into your trucks wiring fairly easily. You would have to do something about the gas guage(thefuel guage in 98+ gas trucks was pcm controlled via the black box pcm, but the 0411 won't do it, so you have to either use the 96-97 s10 fuel guage buffer, or the speedway motors box), but the rest will work as stock. Or you could stick with a 98 pcm, but they don't take well to tuning. Or you can go aftermarket for fuel injection or (GASP), go with a carb. As far as the actual engine swap, it drops right in. If you have a 4l80e I strongly reccomend a different torque converter as the 350 had a fairly loose converter, but otherwise it bolts in directly.
As to mpg, the late model 454 should net in the 10-14 mpg range while the 350 empty should be in the 12-16 range. Loaded down I would think the 454 would do better. Or you can do a 383 swap, use some aftermarket 180 or 200 cc runner vortec heads, marine intake, ls7 injectors, an 0411 pcm, and wake up your small block. You would see a significant boost in torque, similiar milage, use your existing wiring, and have ok power towing(it wouldn't be a 454 as there's no replacement for displacement, but it would havea good bit more power). Other options are a late model lq4 6.0l swap, or even an 8.1l swap. You might could even find an 8.1l out of an 01-02 3500hd that is a drop in swap for your body style. They used the 4l85e, had massive torque, and got a bit better mpg than the 454. Or you can normally find the 8.1l/4l85e combo from people doing duraburb swaps for reasoneable money.
