Hi
This is several months old but i'm posting to mention my interest in the project because I plan something darn near identical and wanted to comment on and ask questions of someone doing nearly the same. Advice might even be too late in part but could be useful to others in the future.
24mpg is easily obtainable bone stock - I have a 1991 TBI 305 caprice sedan that got 22-23mpg and I drove it 77-82mph everywhere for tens of thousands of miles of travel. (pre ecomodder when I was rushing everywhere) Possible a wagon is like a natural boattail/more efficient. Driven more like 65 I could see 24. The LT1 was known for even better mileage (and power at the same time) just for reference.
30mpg should be quite possible. For example looking at 1987 pickups the 6.2 diesel showed 17/21mpg vs the 12/17 of the 5.0 with the 4spd auto - 23% better on highway. (even more in city, 41%) 24 plus 23.5% better is 30. Around town expect 21mpg - I always got 15mpg and I drove light around town.
Rear axle should have zero problem with power - the stock 305 tbi was 170hp 255lb/ft, the typical 6.2 GM's (non hummer) were 130-143hp and 240-257lb/ft. The army version was 165hp and 330lb/ft. Max RPM is 3600rpm and the transmission may shift late without being set up for diesel shifting though. Torque converter may stall higher but still work, it locks up on the highway at least in OD, and possibly in D.
Big Dave - do you have the name of the person who put one in a Fleetwood?
To his recommendation to go to a stick - the older 700 R4's werent very durable. The newer 4L60's (new name for it, with upgrades) and all properly rebuilt 700R4's are supposed to be vastly improved. They last 500k miles in taxi service often after a life of police service. I see no reason the transmission should scatter turning slow - it runs the same speeds with the 305, he hasn't changed any ratios anywhere. I know of no reason the power profile of the 6.2 should kill newer transmissions, only older ones like 82-86 in particular. A T56 is great but probably costs $2000 by itself before the install kit. A rebuilt 4L60 or TH700 should work a junkyard TH700 from 82-86 is the least recommended - they suck. It's not even cooling issues, they just have weak brittle parts. Shift kits and things that make them shift harder could be even harder on it. 87 up chevy fixed the worst issues, incremental improvements afterwards. The 4L60 already in the Caprice is probably marginally stronger otherwise than pre-90 trannies - and should bolt up if the 6.2 has standard small block bellhousing which I think it does, except possibly for shifting at too high of rpm. But it may be the 700 bases all shifting off the TV cable and adjustments there could be done.
If you had to try and swap the engine/trans in a second time would there be things you could do to ease the job? Like where did it hang up besides ABS, power steering, and oil pan? What might make the job easier? I'm mildly confused at the engine mounts, I thought the small block chevy mounts were the same as the diesel ones basically...
Not sure how the trans pack will increase life - if it shifts harder (which most similar things are for) it can shorten life by being harder on it. The 6.2 diesel isnt too much heavier than a small block, 200lbs maybe, among the lightest diesels made, but still heavier than a big block by another 100lbs or so.
Hope to hear how this turns out later.