"C" series cylinder heads were designed at a time when fuel economy was a huge factor in marketing GM full-size pickups. Mostly, this involves precups with the smallest ports. 6.5TD heads could be rebuilt using "C" cups. Dr. Lee's Avant 6.5 comes very close to the ideal fuel economy engine.

Saw a discussion about the NV3550 the other day on TV. This is an aluminum cased 5-speed, rated for 450 lb-ft of torque. This might be a good way to go for the gear-bangers out there, to use for a fuel economy rig.

Automatics with a locking TC come close to the efficiency of an OD manual, but the manual is best.

MP