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a 6.2L diesel only makes a truly lame 240 ft/lbs TQ, no more than an equally lame 305 gasser. The 6.2 feels torquier to drive than the 305, but that's only because the torque comes on quicker and stronger off idle.
A 700R-4 does need to be "built" to survive a 6.2, but even stock 305s break the best of stock 700R-4s, however, the 700R-4 isn't inherently weak, most of the guts are as strong as TH400 guts. The aftermarket has addressed the rest.
Cruise RPM for a 700R-4, 3.07:1 gears, and 33" tires is fine for a 454, the 6.2 probably can't pull it, however.
If you want MPG with a 454, then swap it into an '84-'90 Trans Am. The math says 25 MPG is possible. If I had money I'd do just that, first trying to pass emissions, then trying for 30 MPG.
The Viper V10 started as 488 cubic inches, they gave it a 0.50:1 sixth with a 3.07:1 rear and 26.2" tires. Only a bit of researching uncovered the '87 GM G-body was able to pull similar RPM with similar drag from only 262 cubes. It was a 200-4R with 0.673913:1 OD, a 2.2857:1 rear, and 25.5" tires. So if a 262 can pull that, a 454 should pull much less RPM. Probably need a divorced NP205 to get enough launch ratio with an even taller rear gear, even if swapping a 2.97:1 first, a la Mustang Cobra, in place of the Viper's 2.66:1.
This may all seem a bit off topic, but I think it worth considering what's possible, drag permitting. Or lack thereof.
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