First of all, we're talking about running aftermarket forged rods here, so any comparisons between stock rods aren't really relevant to this thread.

Anyway, the only way to run B18C-length rods in a B18B block is to also use a B18C crank.

Whatever savings you're getting on the pistons and rods over new units will unfortunately be more than superceded by the cost of a B18C crank. Just as a point of reference, MSRP on a new one is $988.02.