Your MPH indicates your engine is making approximately 102 flywheel horsepower. I would forget your carbs and intake, and start with the basics, firing order, leakdown test, ect.
I would, in your case, buy a custom piston, with the metric ring pack, and chevy pin size, and run a 6.125 or 6.250 chevy rod. MUCH lighter than any shelf 340 piston, with superior rings. Jmho
Is the new crank a full radius crank? But regardless, if you sent a bearing sample, then your new rods should be machined to fit your bearings...I can't see a problem
Get rid of the 4:10 gears, I'd suggest 2:94 or 3:23, tune the carb, make sure you have a good timing curve in the distributor, and definitely use vacuum advance, that should net 20s pretty easy
I have a similar setup, 98 5.2 magnum in a 71 dart, stock longblock, cam and all. M1 dual plane, thermoquad, Doug's headers, summit 2.5 exhaust, 904 and a 3:23 sure grip. Gets almost 22 mpg and can cruise cross country with no issues
Nope, car was on slicks, was dead consistent, the gear swap was a experiment to see what the car would do...it was crazy, it ran EXACTLY the same ET as I'd did with the street gear
Your won't know till you run a back to back test, but odds are, you won't see much difference going from 3:23 to 3:73 with your 408.... I have ran a 3:55 gear, swapped in a 5:13 and the car ran the EXACTLY same et
Be pepared for a sea of excuses....... Culminating in the statment "MPH numbers dont mean s@#t" by the OP. His reasoning was it ran in the elevens, so it MUST make 500 HP..... I started out by pointing out he could go alot faster with tuning, but gave up when I reilized he wouldnt understand.
I agree 100% with what your saying...... Its just like trying to convince people on this site that torsion bars and leafsprings work without the need for all this tubular coilover non-sense. Lots of people assume if its "new" its "better" and we know that's not allways the case.
If you MUST do it with iron, I say get a set of W2s The earlier ones had a std LA exhaust pattern, then they had a dual pattern in later years. I have a set on a 340 right now, with regular headers.