Just me, but seems a 454 would be a bit tight for the cowling, may have to make some sheet metal modifications to clear the valve covers up front.
Bigger is not always better, there are a lot of smal bock combinations that will pull like mad and still get decent economy. & if you're more keen on gas than diesel there are some modern engine/tranny combos that would get you lots of power w/o needing 2 fuel tanks to get across town.
Might look at some of the GM Vortec motors, they have a great power range and decent milage on top of that.
The 5.3L v-8 in my '00 GMC is tied to a 4 speed automatic that gives great reliability and grunt. I have moved a good deal of heavy loads with it over long distances and I'm more than satisfied with the performance. The downside is it's not really something you can tinker with if it poops, too much computer, not enough motor!
