... It starts so quickly that it is hard to get a good reading but it is somehwere between 50 & 60 amps which is more than I expected but nowhere near enough to start an 8-92.
I was hoping you'd post the results, because I've been wondering how much difference your 8D's would make over my tiny car batteries. A good bit, it would appear.
I would guess you're not getting an accurate reading if your meter is digital and the thing fires quickly. You might turn the fuel off at the rear switch and try cranking again to get a better figure. I would guess you're somewhere north of the figure you posted. Still, that's a long way from the ~120 or so that I am seeing.
This is why I wrote that a lot depends on the relative size of the banks. My giant house bank, with eight size 8D, completely overshadows the two puny group-65 chassis batteries. They have enough oomph to start the big Detroit, even in the cold, but if they are not isolated, that enormous source, even 20' away over 00 wire, is just too easy to access.