For long distance touring, you need reliability, easy maintenance and modern performance: a no-brainer here, as mentioned already, go for an LS engine: powerful, reliable, repaired by any Chevy shop and a great variety of sizes and trans: from 4.8 litre to 6.2 litre, (even hybrid!), choice of transmissions (manual or auto). They come with power steering, a/c compressor...
Go read the LS1Tech forum, there is an area for conversions, these engines have been installed everywhere! You can get your harness reworked and computer reprogrammed and the whole thing is real easy: swap engine/trans, connect everything and the truck will start at the first touch of the key!
And to choose wich one you want, look up (online or at the dealership) the specs of late model Chevy vehicles (trucks, Corvettes, Camaros) and see the level of performance you need/like.
Unless you tow something really heavy, you do not need a diesel and the stock, period engines are too weak to live happily in modern traffic.
You will probably also enjoy a front clip for power disc brakes and power steering: mid '70s Camaro are great.
