Very simply, which engine do you guys think is the most cost effective to build to "max" power and torque? Now that I own a 6.5L, I have my own opinion on the matter, that for less actual money, you can have the same power in a 6.5L that you can get out of an absolutely blown 4BT... Max power numbers in trucks are close to 300hp and 500ft-lbs for both the 6.5L and 4BT engines, possibly a tad more if you figured out a new mod for the 6.5L, but that seems to be the "safe" max... Max power numbers for the 6BT is obviously higher, but in my opinion should cost more in the long run, especially if everything you do to it is mechanical in nature... My former boss is currently running around 340hp and 700ft-lbs in his 97 Dodge Dually... and I met a Diesel Tuner Dyno guy last year at TDI Fest who's 02 Dodge Dually was puting down a tremendous 1200ft-lbs of torque (sorry, don't remember his hp.) The 97 was all mechanical mods, and the 02 is mechanical mods plus computer mods. The things I know... a 300hp 6.5L custom built engine costs about $10,000 from Peninsular Diesel, and that to build the same motor in your truck it would cost about $4000-6000. A new "Stock" 6.5L engine runs about $5000 in case you needed to start from scratch, and comes with 195hp/???ft-lbs of torque for the 97+ model. That's a good start. a new 4BT costs somewhere in the neighborhood of $4000... it starts out with 125hp/250ft-lbs of torque... enough to move our trucks, but it NEEDS mods to be effective. Incidentals include tranny adaptors, fabrication mods, fitment... Add in all the mechanical mods my ex-boss has on his truck, about $3000, and you are already at $7000 for just the motor and mods, not including the actual cost of installation. a new 6BT is just well... unreasonably priced... around $10,000 if memory serves, and it puts out something like 230bhp/330ft-lbs stock... in the 12v version... add in the same $3000 for mods, and more for fabrication and fitment... So, being that you can get a reliable 300hp/500ft-lbs out of a 6.5L TD, why would you go to a 4BT or 6BT? The ony reason I can see switching to a 4BT is the improved mileage... promises of 30+MPG in a K5 almost made me put one in my own truck... The only reason I can see switching to a 6BT is the possibility of more power later down the line... and I know the used ones don't cost that much, but can you really keep your total cost under $4000-6000 and get 500ft-lbs of torque in a 5.9L? Likey, yes... and the promise of being able to go even further would tempt me, but do I really need 1000ft-lbs of rear wheel torque in a blazer? I think not.