good luck buddy, one of these days you're gonna have a nice surprize!
for many years folks were either using a stock length replacement bolt or the same bolt again with no reported failures of the bolt or the BHJ balancer. So I'd take that bet if you wanna lay odds.
I feel the main reason to go with the longer bolt is to fill the hole in the crankshaft so that any stress is carried across a greater area. Long term it's got to be better for things. But since the bolt
is not a primary retention method for the balancer, it shouldn't really have that much stress on it with the BHJ unit if it's properly machined for the interference fit. (Interference fit keeps it on the crank. Keyway keeps it from spinning on the crank, bolt just keeps it from walking down the crank due to vibration.)
So since you need to buy a bolt anyways, why not buy the one that's creates, effectively, the same amount of engagement with crank threads.
One note here, do not use the retaining bolt to press the balancer onto the crank. There is a special tool for that. Using the bolt, puts excess pressure on the threads in the crank as you turn the bolt. The proper tool has fine threads and a fixture designed to press balancers on. Use that.
And finally, make sure you get the balancer honed to the proper spec. Crankshaft variances are significant in our cars, so make sure it's not too tight, or getting it on may be possible, but getting it off could be impossible.