My First time engine replacement was with a mechanic who didn't know what he was doing. He put a car 454 in it instead of a truck one. The engine blew 18 months and 3500 miles later. "Out of warranty", he said. Next time was and awesome mechanic. They rebuilt the engine. I had the transmission rebuilt the first time. It was the Heavy Duty 400. A very good transmission, albiet a 3 speed. I average 8 to 9MPG. You have to install en engine and transmission that are synchronized to work together or it won't work right. There are "Brain"/module issues and wiring and other complications to prepare for. I've thought about it but it's not worth the dough for the savings unless you are seriously considering traveling a lot.
The best factors to consider are:
Cooling, like extra airfow around the engine, good auxiliary fans for cooling the oil and transmission fluid, Quality coolant,
SPEED! Slow Down! 55-60 is the sweet spot for those rigs. The 454 runs hot anyway and is brutal on exhaust manifolds. Quality headers can help some but speed really makes a difference. After the engine rebuild I had to keep it under 60 for 1500
miles. I like it now. It's good for my head too. Relax and enjoy the trip.
BTW the engine goes out the driver's door. Chair comes out and the door panel too for clearance.
If I could I'd see how to put the 8.1 in it with the proper trans. Could mean some custom drive shaft work and wiring.