It wasn't custom shaft what was installed. Marlin's used a Chevy v8 clutch, longer pilot bearing and AR5 bell housing to make the R150 work behind the Isuzu engine. The R151F out of the Toyota trucks WILL NOT work without a complete tear down to replace the input shaft. This is due to it using the short shaft and Isuzu using the long shaft. I really don't feel using the R-series for a swap is worth it. Most have a lot of miles and as such will be quite worn out when you get it as opposed to an AR5 which can be found with well under 100k miles. When I pick my AR mine up in Feb/Mar I'll check the rear output shaft and call Marlin crawlers up or possibly go to a few junk yards and just buy a t-case from a Toyota to see if it will work.