Aah, hate to have to be the one that tells you, but 400 SBC's ROCK. The only thing better is a bigger SBC, which is done all the time. Why? Because it makes more power- EVERYWHERE! Not just down low, but all the way from the basement to the ceiling.
A good 400 SBC makes more HP AND torque at 4500-5000 RPM than a 300 CID SBC will anywhere in its powerband! At its peak, the 400 SBC will be making tons more of everything.
If you think that a 3.75" stroke is for a "tractor"
, I would hate to think what you'd say about a really big stroke- but it's a moot point. You have your mind made up. You're certainly not the first guy to come here w/a "world beater" idea that was less than... stellar, shall we say?
Take a look HERE
. This was written 13? years ago. So, you see, there is nothing really "new" under the sun.
YOUR "problem" is what you want to use the engine for/in. A small displacement, high RPM 'drag' engine is not what a van that weighs 14,000
(!!!) lbs. needs to get it down the road!
You also keep forgetting that you have to get that 7 TONS
of van moving from a dead stop. That's a big part of where your idea of fuel economy will fall flat on its face. The other part is anytime the engine's required to make power, you'll have your foot solidly to the floor, w/all circuits flowing as much fuel as possible- or you'll burn pistons- or you will not accelerate- or even hold your road speed at any type of grade, w/o down shifting and revving hell out of the 300 cid, 3" stroke SBC. Even then- if the net torque isn't sufficient- you will slow down, not speed up.
The balancing act is to find enough power that you don't run the engine at the HP peak, but closer to the torque peak. The closer you come to running at the HP peak, the worse the potential mileage gets.
I'll close w/another link to an Isky Cams article where rod length (among other interesting subjects) are discussed. ENJOY