I didn’t realize that they are making the marine manifold with cats on them. If you do the math for the 3.8 stroke a stock block is too short to put a long enough rod in it to achieve the better rod ratio. The 3.75 stroke w/a 6” rod gets you close to stock ratio of a 5.7 and unless you can find that intake on a deal you’d be better off with a aftermarket one IMO. GM had a lot of trouble with the 400 sbc the bores wore excessively piston pin failures that was because of the rod to stroke ratio wasn’t enough. It’s also why there is a tail deck block because once you calculate the rod ratio you end up with a longer rod and then you don’t have enough room on the piston for the ring set w/o going into the pin bore. The taller deck gives you the room for the correct piston and it also to an extent changes where the engine makes power and it also looses efficiency with a shorter rod. The guys running big cubes don’t worry about a budget or how long the engine will survive. I took the stock L31 intake and ground it more or less flat on the bottom and filled in the low spot opened the ports up to the gasket I got from edelbrock and pinned it so it couldn’t slide all over. Plugged the egr off. I didn’t take any pictures because I don’t know that it’s going to work that well or not and may eventually have to go aftermarket. The biggest issue with an aftermarket intake is the distributor cap plug wire terminals line up with the fuel rails luckily MSD makes a distributor that uses top terminals with a cam position sensor to run the gm ECM. Do you plan on dynoing this when it’s finished?
3.80" with a 5.7 rod is how GM builds 1,000s of them.
I have also put a 3.875" and 6" rods in a 5.7L block at 0.030 its 395 CID from memory. Very common build in the Gen 2 LT1 world. A 395 LT4 I built 10+ years ago has 150,000+ street miles on it at 550+ hp. Still going strong and the only problem it has had was small base circle required failure prone link bar lifters that have had to be changed like twice. I wanted a normal base circle and OE style lifters in my 383 or it would be a 395. The link bar lifters are JUNK compared to the GM setup.
A stock 400 had 5.565" rods. GMs only small block departure from a 5.7" rod except for the 94-96 L99 4.3 with its 5.94" rods.
The 6" rod with a 3.875" stroke still has a better rod ratio than a 5.7" rod 3.75" stroke 383. A 6" rod 383 has only a couple decimals worse rod ratio than a stock 350.