> by having the longer rods, the ratio of rod length to crank
Rod-ratios have effects. But not huge effects on torque. Apparently some NASCAR teams use different rod-ratios for long or short tracks; but this is hair-splitting in an endeavor where 1st and 4th can be a hair-split apart.
Rod ratio has major effect on cylinder wear. Though for a given stroke in a given block, not huge. Ideally you want a medium long rod, but that makes a tall heavy engine.
> 530ft lbs, @ low rpm, with a stock top end.
You can do that with narrow cam timing. Many engines have "8:1" compression ratio (it is actually the expansion ratio that builds torque, but ER is approximately the same as CR) but close/open the valves far up from BDC so effective CR/ER is more like 6:1. This gives more top end but hurts the bottom end. Since you can always gear-down to use your top-end, mega-bottom is a special-application trick.
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> The big-block Chevy is a bit heavier than my Caddy
OK, that makes sense several ways.
The instigation for the BB Chev was NASCAR. Mopar had real trouble casting B-blocks which would hold together under Hemis in NASCAR. (Ford had less trouble because the FE heads wouldn't breathe.) GM knew this would be an issue and put lots of meat in the bottom. NASCAR is weight-conscious but not weight-crazed. (Yes, GM pulled out of racing just before the 396/427 was released.)
The cover-story for the BB Chev included trucks, which again may need a beefy block, and are not adverse to some weight.
BB Chev is 1965, when Thin Wall casting was common but maybe not mature. And if successful, the BB Chev would be produced by the millions, so ease-of-casting was vital. Extra thickness of iron casts easier.
Caddy's 472/500 was an all-new casting, in 1968, when Thin Wall was maybe better controlled, production was small and Caddy could afford a little care in casting. The Caddy was NOT going into trucks or racers. While the 390 was sometimes rated as high as 4,800RPM, and the BB Chev sometimes for 6,400(!)RPM, the 472/500 was rated "only" 4,400RPM. And worst-case would never run over 2,800 for more than a few minutes.
> If it gets a bit over 400 HP I'll be happy enough.
Early Caddy 500, with 10:1 compression, are rated 400HP stock. True, this is old-SAE, which is unrealistically optimistic. Between 1972 HP-rule changes, smog, etc, HP rating fell to 180HP, and then they de-bored/stroked it for a 425CID because NObody wanted to buy a 500.
Good compression, more cam, free exhaust, check for port casting slop, let it past 4,400RPM, there should be 400 honest HP easy. But note that you can't use 400HP in a 2WD 3,900 pound pickup at any legal speed. You may be able to smoke rubber all the way to 75MPH. At say 50MPH you only "need" 300HP for good acceleration. You can use the full 400HP from 90MPH toward 130MPH. You may need to down-shift, or run 2.8:1 gears, to beat 115MPH. With Caddy's extra-tall gears, the car would top-out at maybe 110MPH, running a not-high RPM, which it could hold all day long cris-crossing Nevada say.
> would not be a very good daily driver carb in my opinion. No choke provisions
Bosh. I drove the Cougar though a couple NJ winters without a choke. The AutoCrap carb got worse. I got a good price on a "race only" Holley 2bbl. (There's a short-track series which has a 2bbl limit.) It tuned right up for clean economical street use; the Holley is a good carb. And it actually had a choke, but manual, and the Cougar didn't have a choke cable. Yeah, easy to add, but that fall I just pumped the accelerator pump, got ignition, half-fast idle for a minute, and drove away. The un-choked Holley ran SO much better than the AutoCrap ever did, that I didn't got around to installing a cable for a while.