I have an E350 Centurian, (modified van/pickup) which I tow a 37 foot fifth wheel. Came out of the factory with a 460 ci, holly 4bbl. Made many modifications. Msd timing control, dyno by Larry Garrity, early model cam... etc. The single most effective change was Holly Pro-Jection. The world got very flat! Scrap that carb idea and go with the holly Port injection. Use the 400 Ci model to get better atonization (unless you race at hi rpms, I kinda doubt it with a horse trailer). Installation was somewhat complicated by comparison to just swapping out a carb, but not really too bad. Holly did a good job of supplying all the stuff and the manuals are good. Took me about 5 hours. Been running it for about 6 yrs now, no problems.