Bronze 68, I was there when we dyno'd that engine! all BLACK and SILVER! Glad it made it "home" ok!
Either will work fine. Look PEAK HEADER TORQUE is a minor measurement which tells you were peak "port torque" occurs in the primaries given the bore area. smaller will give you more 3000 rpm boost, larger more after 4500. That's all it is, no more, no less. So its a"secondary" optimization equation, not a primary.
I thought about "optimizing" mine to 1.875", but Mark said 2" are fine, actually preferred FOR MY APP ( maybe as it's a clutch, no stall) So i had my 2" Hookers coated a "TURD" time! In White! Smaller coated Hedmans NEW would have been cheaper.
I cant speak to lack of TQ, as you cant go WOT on street rubber with an MJ mill. Not with my race Holley, maybe with EFI you can.
They actually use the math in this article;; Math dont lie, PEOPLE do! The Correct-Sized Headers Will Give You the Best Performance
Based on that 2" works for an MJ468. Smaller only means you'll shred more rubber, earlier. But if you already had 1.875"; I'd run 'em. No headers yet - WHATEVER FITS BEST! I 'd say. But at least using the math in the article, you can bust out a calculator ( I'm not an engineer, but a numbers guy) and know where the 2" versus the 1.87" put their peak header TQ ( approximately 4500 and 3900 respectively on a 468).
Your engine likely shows peak TQ at 4400, as mine did with the same cam. So, given the math, you can dial in the headers to give you peak HEADER TQ right above the engine ( as I did, as I'll drop the clutch at 5200 to HOOK up at 4200...) or to come in just under the engine's 4400 rpm peak TQ at 3800. ( if for example you were using a 3500 or less rpm stall, flashing to the peak pipe TQ - this makes sense)
So, as Mark said, either will work, the math shows what you might expect from an A/B test.
PS I had my system pro -designed ( Dr Gas engineer), the 2" hookers feed into a 3.5" collector; then thru a 3.5" to 3" Dr Gas X pipe to 3" head pipes and 3" Magnaflow 12578s. 2.5" Torque tech ( not in biz anymore) tails finish off an engineered system for 500-700 hp, meaning it should run same with open headers.