Ending the season early with a broken crankshaft
Couple of my friends have a 604" 9.8" deck BBC, the rods in that are 6.385". It made 1320HP on one carb. I may have misread your posts but it appears you need pistons rods and a crank? Or were you just going to buy the odd number of pistons and rods you'd need, reusing what's good of the old stuff? I have good respect for your engine builder and I'm not sure if this goes against the grain of what he's advising or is it from you? I know you can get some if not all 4.25" cranks to balance after you cut the counterweights for a 6.535 rod on a typical 1.3 c/h piston WITHOUT adding mallory. Now if you had a stock block, I like the longer rod for the reason you stated.
Oh we are certainly aware of the amount of BBC out there with short decks, short rods, and make big power. No disputing that. The rods I will be changing because they are chevy journal and chrysler width. So my option there is to buy another crankshaft that is chrysler journal and chrysler width, then have the crank cut down to 2.2 journal to use 6 of these rods and replace 2 bent ones. Since my choice for a replacement crankshaft and rods is molnar, their cranks come in chevy journal and chevy width. They don't offer one as chevy/chrysler. It is either chrysler/chrysler or chevy/chevy. Pistons, I may replace the 4 that need replaced, I may buy a whole new set. That is up in the air right now. My builder and I looked into shortening the CH on my piston in order to swing the 4.25 stroke and keep the 6.76 rod. It is possible, but may be a game we just don't want to play. Even if I shortened the rod, to pull off 4.250 stroke, my piston would still not work. My compression height is 1.125 and you are right, I would need 1.300 compression height with the 6.535 rod and 4.250 stroke. Although there are tons out there who do it and are successful with it, it is just not something I am interested in. If I went to a 540, it would be with a shorter CH piston (1.125 CH currently down to a 1.075 CH), and maintaining the 6.760 rod. That would do a couple things, first I would add compression to the engine going from 14.6:1 to 15:1 keeping the same Dome we currently run. Second, I would go with .9mm,.9mm,3mm ring pack over my current .043,.043,3/16 ring set. Third, it would lighten the piston even more running less CH. Would it make more power? I am sure it would. Is it worth the effort? That I am unsure about. The longer rod definitely has an advantage as you are aware. Shortening the rod and adding compression height to the piston would not only make the rod angle worse, but also add weight to the piston, so at this point just something I am going to stay away from. Even in my aftermarket Indy block, I prefer the longest rod possible.
For the sake of keeping it simple, I am leaning toward just staying with the 528 combo.
Sorry for not answering the question clearly. What you said is correct, my builder is advising against the shorter rod for the reasons listed above.