I would go with the 454 based on the displacment,but dont be afraid of the tall decks,they were made for truck much bigger then yours.A tall deck will bolt right in{it is only a little under a half inch taller then a regular bigblock},the thick 4 ring pistons of the tall deck allow it to run cool and damage free even when pulling heavy loads up steep hills with severe detonation.The short stroke of the 366 and 427 allow it run run fine at somewhat high sustained rpms such as would be encountered in a big straight job truck or school bus where typical gearing is the equivelent of more then 5:1 in the rear axle{A big truck would have more like 8:1 in the rear but it would have much bigger tires},and the small bore of the 366 combined with the abilty for it to rev 3000+ rpm on the highway for sustained periods of time make it a great choice for heavy pulling.The kind of grunt work that would blow the tops off a set of 454 pictons wont even make a 66 rattle a tiny bit.
It all depends on what you want to do with the truck.If you are building a performance truck for hot rodding then a 454 might work fine,but if you are building a truck to pull a 30foot enclosed car hauler across country then a 427 truck block would be a sweet find.Good luck.