It seems to me that you want to build the main t-case first. There are a couple of issues though. If you mate to a 420 you need the 10 spline Marlin input. then later swap it to the front crawlbox. Yes a 23 spline is what you need/want but they can be purchased brand new (good price from Marlin) so that opens up the t-case options. If you eventually want a dual ultimate the 4.70 will help your gearing right now Either way you are looking at a new input gear.
The way I would do it is to get all the pieces and do it all at the same time.
Scott
