The 5.3 would be a good choice and is fairly easy to find for around 500 bucks. But if you can find a 6.0 either a lq4 or lq9 you would like the power much more. They can run around 1200 for a nice one. But it's worth it.
The LQ4/LQ9 would probably actually get better than the 5.3 in that big truck too.
1969 Chevy RCLB C10 350/TH400 SOLD
2007 Chevy RCSB 4.8 4x4 LS SOLD
2008 Chevy RCSB 5.3 4x4 LT SOLD
2010 Chevy CCSB 6.2 4x4 LT SOLD
2005 GMC CCLB DRW 6.6 Duramax 4x4 191,000 and counting
2013 FORD CCSB F350 6.7 Powerstroke 4x4