My truck is getting a 6500 Optimizer, most likely the naturally aspirated version, as soon as I have the funds to do so. For the interim, we picked up a $300 HMMWV takeout 6.2 as a spare to keep a truck going in a pinch. $4500 buys a NEW 2000-2008 model 6500 N/A, not much of a power boost (173HP, 305 ft-lbs), but the casting issues on the block are improved, fuel economy improved, etc. Turbo drop-ins are in the $7000 range (250hp/550 ft-lbs) and rebuilt 6.5Ls are in the $3500 range.
Hope that helps.