If you're using a stock Mopar rod journal size crankshaft the 6.123" length rod is all that's offered.
If you're using a crankshaft that's made to use a 2.100" Chevy rod journal you can get them in the following lengths.
6.125" In Stock
6.250" Back Ordered
6.300" In Stock
6.400" In Stock
6.500" In Stock
Price is the same for any length rod listed.
Molnar does not offer any crankshafts with more than 4.00" stroke for a SBM.
Tom