I have heard people mention that you can run a SBC 6.200 rod with 4.125 400 SBC pistons for 362 Cu In, which replaces the Olds rod and is the same as a 425 Olds piston swap. Does anyone know who has done this or is this just a guess by others?

I did talk to BTR (Bill) this morning who mentioned that they downsize and widen their Olds 350 crank rod journals for a SBC 6.00 rod, but that he hadn't used the 425 Olds or 400/6.200 SBC piston/rod combos in his Olds 350s.
Seems like a no brainer to run the SBC 400/6.200 if the right Comp Hgt is used and that the Deck Height is in the hole .002 or more.

Bill's option was to use 4.065 CP piston and a 6.00 SBC Scat Rod. His balanced rotating assembly with your crankshaft (pistons, pins, rings ($700), rods ($350) and crankshaft work ($200) isn't too costly for a very good setup.