get a 435(or t18) with a 205 attached. id avoid a t18 with a 208 if you want to go the doubler route,as the 208 will have a different stickout length for its input gear,and therefore possibly either a shorter tailsahft on the tranny,or a longer adapter.
i have been told that the ford 203 will have the same stickout for the input as the 205,and the same round NP pattern. im also told it doesnt matter if it comes from an auto or a manual,it will bolt on where the 205 was. this is just what ive been told,ive never laid my own eyes on a ford 203,so i cant really say for sure.
other than that,you have it figured out. youll disassemble and discard the chain section of the 203,and whomever you buy a kit from will either cut down and spline your shaft,or send you a shaft,and take yours as a core. as such,the range box will need disassembled to swap in an appropriately splined output shaft to slide into your ford 205.
the doubler adapter bolts onto the rangebox in place of the chain section,2nd t case bolts to the adapter. rig up the shifters, make some driveshafts and hit the trails
as for whose kit to use,i cant remember whose the guys were the most impressed with. imsure desertbeast or blt2krl will chime in on that one.