So, if you are going the SBC route, don't!!!
This is only half a joke: as good as the SBC was, it is a very old design, improved over the years. If you are going to do so much work, if you need easy power, reliability and good mileage, us an LS-family engine. These are the modern, current "small block" available in all Chevy trucks/vans/Camaro/Corvette since the late '80s/early'90s. They are very modern engine, very good engines and they are a bolt-on wherever a SBC would go (with just an aftermarket adapter). They come in a variety of sizes, 4.8, 5.3 (one of those would be the best for your project), 5.7, 6.0, 6.2. 7.0 liters... They are powerful and reliable, easy to maintain and parts are everywhere!
Many people are scared of the electronic engine/trans management, but help and parts are very common: to start, go visit
www.LS1tech.com, they have a section called "hybrids" for vehicles that did not come with such an engine, and there is vast knoledge available there. Many shops/companies specialize in these engines, you just do not see them mentionned here! I would not be surprised to find dedicated headers or bolt-in kit for your truck, even. I have used a company called Speartech to reprogram my computer and modify my harness. It was my first engine swap ever and went very smoothly!
They are also very very common and very cheap in junkyards.
As for mileage, there is no miracle, our old trucks are not that heavy but are so un-aerodynamic that good freeway mileage is close to a dream...
The dream engine for my '57 Suburban? a turbo diesel from a Dodge Sprinter van, but I do not have the budget or skills...
Congratulation on driving your truck as a daily-driver, but aren't you worried about quick damage from road salt and grave? Mine is only a 3-season daily-driver, winters in Calgary are too hard...
