If you do go with a big block I would for sure go with a coil over front end, that way you can get enough spring rate with the heavy front end, plus it will save you a lot of headaches trying to fit the big torsion bars and headers in the car.
I for sure wouldn't. You can get more than enough spring rate with torsion bars. Firm Feel has 1.18" torsion bars on the shelf, they have a 370 lb/in wheel rate. And they'll make you custom torsion bars if you'd like, all the way up to 1.24" is possible with the A-body torsion bar anchors. Not to mention it's a 400 stroker, so it probably has aluminum heads anyway? Which means that engine isn't much, if any, heavier than a stock small block with cast iron parts. Like the 340 in my car which still has iron heads on it.
For your AutoX goals you don't need an aftermarket K frame. You can do everything you need and more with the torsion bar front end if it's set up properly. AutoXcuda does it, Tomswheels did it with his Valiant and has even outpaced Mary Pozi's 500 hp autoX monster camaro at an event, despite being down 150 hp. Tim Werner's "red brick" valiant has lapped at 160 mph with torsion bars and leaf springs at Portland International, and put down a time faster than a C06 Corvette. No kidding. LilCuda owns that car now.
Also, for the
"coilovers are the only way to go fast crowd", check out this Hot Rod magazine article. You'll see that the Hotchkis taxi, a '70 Satellite 4 door, ran over a second faster on the autocross than Wracks's duster did. Wrack71's duster has the full RMS treatment, and he's spent the time setting it up too. That's a
big car with torsion bars and leaf springs coming in faster on an autoX course than a fully coilover converted duster. Not only that, the Taxi was the 2nd fastest car on that autoX, faster than all but one of those other fancy rides. And that Satellite is at a huge disadvantage, pun intended, because of it's size. Wheelbase and weight are very important in autoX. Here's that link-
2013 Muscle Car of the Year - Popular Hot Rodding Magazine
My Duster is set up similarly Steve's (autoXcuda) and Tom's (tomswheels) cars, my specs are-
-1.12" Firm Feel torsion bars (300 lb/in)
-Afco leaf springs, 120 lb/in with Afco leaf spring sliders
-Hellwig 1 1/8" tubular front sway bar
-Hellwig 7/8" rear sway bar (e-body application)
-Hotchkis non adjustable Fox shocks
-13" Dr. Diff cobra style front disks, 11x2.5" rear drums
-16:1 flaming river
manual steering box
-18x9 front rims with 275/35/18 BFG KDW2's, 18x10 rears with 295/35/18's same
tires
-lowered to ~1" clearance from frame to .375" thick LCA bumpstops, ~5" from ground to K-frame, 4" to the Dougs header flanges
-Milodon road race oil pan (baffled, trap doors, 6qts)
-3.55 rear gears with 833, 68-70 B rear with 1/2" spring offset
-frame stiffening: subframe connectors, tubular
radiator support brace, torque boxes, "J" bars tying firewall to front frame rails above
radiator support brace, shock towers tied in and reinforced to J's. Stock LCA's boxed, K member seam welded and gusseted
-currently running non adjustable tubular UCA's and stock LCA's, soon to be replaced with Bergman double adjustable UCA's and gusseted QA1 tubular lowers for easier alignments and additional frame to LCA clearance for further lowering.
It's a heck of a set up, the car handles VERY well. I'm always working on the darn thing though so I haven't been to any events to
really put it to the test, but there's no reason why it wouldn't hang with any of the other cars I've mentioned based on its set up. Only difference would be the tires (need much stickier tires for that!) and of course the driver, as I'm sure I'm not nearly as capable as any of those guys. But that's not the cars fault. ;)