Several years ago I swapped out my TBI 454 in an 89 V-3500 for the HT502. Best thing I ever did. It would pull and pull and ran on pump gas. The downside is it cost around 6k for the motor from GM. The best it would do for fuel mileage was 9mpg. I ran summit headers on it, an edelbrock 1407 carb on an air gap intake, jacobs ignition and rapid fire plugs. I had a friend use edelbrocks multipoint conversion for the tbi and it ran great and pulled real well for what he was using it for.
40 feet of 5er is alot of weight, have you considered swapping to 4.56 gears in the rearend and using a gear vendors overdrive to split the gears and still have a decent cruise rpm?
Good luck and keep us posted on how it turns out.