1. Do both trucks have the 4L80E? If not, you'll be swapping transmissions and driveshaft yokes, too. Even if they're both '80Es, you probably want to keep the big-block transmission/torque converter with the big-block engine. Are both transfer cases the same?
2. Does the Crew Cab truck have the 10.5" full-float rear axle? (Or bigger.) If not, you'll be swapping axles, too. (I think you're likely OK there.)
3. What radiators/coolers are in these trucks? You'd want the largest-available rad with the Tonner/big block. Look at the add-on trans and power steering coolers, oil cooler in driver's side rad tank, verify the oil cooler hoses aren't leaking. Does the big-block truck have an electric fan in front of the radiator? Mine does. Good time to flush the coolant on both vehicles, pull the block drains to remove sediment and old coolant, fresh serpentine belt, fresh PS fluid.
4. You'll be swapping--or reprogramming--the computers. Maybe the underhood wire harness, depending on how it fits the accessory drive and various engine sensors.
5. Do the A/C compressors mount in approximately the same positions? In other words, can you leave the A/C system pressurized, so that the A/C compressors stay with the vehicle? Or do you need to swap AC compressors, hoses, etc. also.
6. I bet the exhaust systems are radically different. The big-block truck probably has two catalytic converters; the small-block truck may not. I bet you'd need to swap everything in front of the muffler. Good time to screw-in four new O2 sensors. The EGR tube from exhaust manifold to intake manifold is not reproduced. DON'T screw that up.
7. No promises on the fuel plumbing. Maybe you'd need to swap the fuel supply and return tubes.
8. Pay some attention to the torsion bars. The big-block is going to add weight to the front-end. When the engine is out, perfect time to rebuild the front suspension/steering.
