Project Black Betty
Swapping the rear end is going to happen whether you have a turbo /6 or a v8. A 7.25 will not with stand the power of a turbo very long or at all really.
I agree thoroughly!
Swapping the rear end is, however, subject to some interpretation.
I have a 1972 V8 A Body with a 360 Magnum equipped with a Vortech supercharger that I drive on the street with SOME trips to the drags for grins. It runs mid-11's (at 3,300 pounds) in the quarter at about 118 mph. Does duty as a grocery-getter.
I have a 4.10 gear for it with a Sure-Grip that works well in the traction-enhanced environment of asphalt + VHT, (plus some 9" slicks,) that it sees at the track. That's about as good as I can do on the gear, as this car has the stock hydraulic-roller lifters with not a whole lot of valve spring pressure, which limits my redline to a little over 6,000 rpm.
It's not a "rev-happy" small block at all... :(
I experience about 3,500 rpm at 70 mph on the highway with that rear gear, using 28"-tall tires. That 3.56"-stroke small block doesn't much like extended periods at that rpm, given the valve train. Not that it won't do it; it will, but it's not what you'd call "relaxed" at that rpm.
I also have a 2.73:1 third member which I swap into that housing if I am planning a trip of any distance at all, that would involve highway driving.
That third-member gives me a more-relaxed 2,330 rpm cruise for the freeway, at the same 70 mph.
Unless you want to engineer an overdrive of some sort for your warmed-over small block, you're probably going to have to something like I did; an 8.75" rear makes it easy and quick to swap ratios.
The turbo slant six is a weird machine that likes to be "held back," for building more boost, so in that vein, the word I get from the two fastest turbo slanters on this board, tells me that a 2.73:1 final drive, works best for drag racing for them. Go figure...
They both run that ratio in their approximately 500-horsepower cars, with good results. They even claim that 2.73:1 ratio works well for their 60-foot times...
That low-numerical ratio allows them to use the same gear for highway cruising AND drag racing, so there's no necessity to have the capabilty of quick, easy gear changes, such as the 8.75" unit makes possible.
The fact is, 8.75" A body housings have gotten relatively hard to find at a cheap price.
The far less-costly 8.25" units that come in some of the '73-and-up A Bodies are plenty strong for a turbo slant 6, and usually, are fitted at the factory with a ratio in the mid-to-high 2's... so they are probably going to bolt in more cheaply that the drop-out housing option (the 8.75" unit) plus, you only will need one gearset for that rear end; the one that it comes with. Another plus is, that I THINK those '73-up 8.25" rears all have axles that are drilled for the 4.5" bolt pattern; not positive about that. ut, I am pretty sure that there was
never an A-body 8.75" rear with a large bolt pattern. That is a small consideration, but just one more thing you won't have to deal with if you go the turbo /6 (8.25") route.
I believe you're also right about the fact that it will take some tuning to get a turbo slant 6 onto the "good" 12's, unless you spend some money on pistons and rods (forged) that will allow you to crank up the boost.
Tom Wolfe's 3,300-pound '70 Dart ran 12.95 in the quarter with an almost completely-stock 225, using a junkyard turbo off a G/N Buick, but he had the boost cranked up to around 20 pounds; not a good way to enhance life-expectancy on stock, cast, pistons... LOL!
Ten pounds (boost) will probably prove to have some longivity, but that won't get you into the twelves...
Choices, choices, choices.... lol!
Lots of luck to everyone concerned!!!
Bill