The only thing you need to know is that the Rear Transfercase MUST be a top shift style, since it will bolt behind our adapter plate.
If this forward shift t/case of which you speak of is your donor case, then yes, it must be converted to a top shift style, no exception.
These gear drive transfercases last a VERY long time if properly cared for. So it really does not matter if you choose to turn your current transfercase into the Crawl Box or your donor transfercase into the Crawl Box.
But since your current transfercase is already mated to your transmission, I would just recommend converting that into your Crawl Box, and then using your donor case as your (2nd) transfercase (which will receive the drivelines).
Generally speaking, 21-spline setups are not recommend for drivetrains that are behind an engine of apprx. 170 ft-lbs of torque or more, such as Turbo R-engines, 3VZ-FE (3.0 V6), 3RZ-FE (2.7 4cyl), or better.
We have seen 21-spline setups fail behind stock 22R powered trail rigs, but this is very rare and certainly done with extreme prejudice towards heat treated metal components. In other words, you would have to drive extremely hard and very rough on your rig in order to break a 21-spline drive train with only 120-ish ft-lbs of torque.
I see more 21-spline couplers stripping out then I see 21-spline inputs failing.
However, if you plan on installing our 4.70:1 gear set in the back case, with it's Total-Spline™ input design, we have NEVER had a single 21-spline 4.70:1 input gear fail, nor have we seen a 21-spline coupler with a Total-Spline™ input gear strip out! It is a far superior gear that is a very strong 21-spline candidate and has solved the issue with stripped out 21-spline couplers.
Regards,
BigMike
