I'm not trying to bash your decision by any means, just throw out technical merits of doing things with different pieces which is the subject of the original post. Since I obviously have a bit invested in the accuracy of the technical discussion, I just want to make sure it's all clear.
Durability wise, you wouldn't break a 203, that spur gear drive is extremely solid. We've had guys breaking outputs on the 205, one (ever) broken Doubler shaft, and one 205 case that ended up in 3 pieces and never a whimper from the 203 gearbox. One of the solid points of the 203 is it's strength and durability. If you can get your 'wheeling done with an atlas (given that the 205 has a higher torque rating and the 203 will stand up to a 205), the 203 would be fine.
"new" vs. "cut apart old stuff"? 1st, it may just be terminology but nothing is cut on the 203, it's a 100% bolt together process, the 203 starts as a bolted together transfer case and our kit bolts into factory points. It's important to clarify this since everything we put down here is available for searches for years to come. There is a variable in the condition of the 203 gearbox you start with though we seldom find one that's completely unusable. If you want a warranted part, we can build a gearbox that way, and even with having to buy both transfer cases, you'll rarely spend over $2400. If you want to do the work yourself, you're obviously responsible for that work, that's part of the fun (?) of doing it yourself.
Really, if you assume you're not going to multiply the input torque into anything less than a 205, (lets leave the atlas vs. 205 discussion alone, it's been covered already) and assume you're going to upgrade the 205 to the 32 spline input before you break it, it comes down to:
cost: already covered
gear ratio: 2.72 vs. 2:1
Weight: ? I would think the 30# guesstimate is about right
Durability: probably a tossup but I might give an edge to the 203 since it does have fewer moving parts. It's probably not a significant difference overall.
Strength: (different than durability) probably not a significant difference. I know the 241 planetaries have a high torque rating, I wonder if that's rated in low gear?
Length: somewhat comparable depending on the application.
Nashua, I think the length example you were remembering was either a GEN1 or a jeep setup we put together that had a 350, ranger OD, SM420, 203 gearbox, then the atlas. You can see why the shifters could end up in the rear passenger area.
