Just keep it simple and go with the 350! There is NOT much difference between the actual hp of a 350 and a 454 for your use. Like the man said... the damn 454 uses so much gasoline it makes any small amount of usable torque priced too high.
As for the older diesels... leave them alone. They were not dependable! the overhead on these engines is waaaay to high.
For a vehicle with TOW in mind and nothing else I would go for the 350 and gear her down so she would tow the daylights of whatever was hooked up to her.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />