I've been watching this thread from the beginning and was reluntent to weigh in on it but, but now it's time. First of all the cast steel crank is junk, and all of the MFG'er reconagise this, they say don't turn it over 6500 RPM and with good reason it runs around like a jump rope (flex's) and will eventually beat out the main bearings.
Secondly the stock 400 crack is a Nodular iron crank and is much less flexable and stronger. If you have a large C'clamp around you can tighten it until is distorts/bending ugly but it won't break they are also made of nodular iron. A couple yrs. ago i built a cheap 388" motor for a guy (649.00 rotating assm) cast steel crank, SIR rods, Hyperutectic pistons) on alcohol. It ran 6.29@112 in a 3,000 LB Camero, he was spinning it 7600 RPM with 5.86 gears. At about 50 passes it broke a valve head off. When the engine was pulled apart, it had also beatout the front 2 main bearings and broke the 2nd main web all the way up to the lifter bores. All this due to excessive RPM's O'h he had fun, but it was not repairable.
I guess i'm saying it's flip of the coin, the cranks will work within reason, but i would rather have a stock GM 400 crank anyday of the week than the cast steel crank from China.
JMO
Zip.