Tell me why it wont work - "homemade" auxiliary tr

Long story, but what would be wrong with using an older "divorced"transmission like a NP435 or clark 285 as an extra splitter behind a regular transmission to get some extra rpms? I know there are new kits and old Brownlites out there, but having trouble finding any in my price range and location, and I already have the extra transmission from a different project. I was thinking of building an adapter for the splined input shaft to hook it to a standard Ujoint yoke, and turning the add on transmission backward, so that the original input would be the output. With both of the transmissions I have access to, high gear is direct and 3rd in the np and 4th in the Clark are something like 1.3-1.5 to 1, so I would essentially be adding an overdrive. I have an older twin screw grain truck with really slow rearends I was wanting to speed up.Im sure there are better ways to do it, and would probably be simpler to get another truck, but I couldnt help but wonder... Thanks in advance for any wisdom here!

running helical cut gears reversed with any load is bad. normally the torque pulls the gears together. reversed the force pushes the gears apart. the case splits since its not designed to withstand that force. old straight cut gears can be run in either direction. if your running a gas engine grain truck gearing wont really help your going to run out of power before you see any increase in speed.

We built a few aux gearboxes from old standard transmissions years ago, worked somewhat OK but noisy, required a lot of effort to make them function at all and in the end more costly than the add in gear sets between bell and existing trans. Unless you can actually buy a fully divorced box there is no clean way to adapt a u-joint yoke to a input shaft and keep it true enough/balanced enough to not vibrate the rest of the drive train to death. Anything and everything we built ended up relegated to field work never put on the road for that reason.

not too many guys can drive a set of sticks any more, or have even seen them

4th in the Clark are something like 1.3-1.5 to 1, so I would essentially be adding an overdrive.......... so why not just put a transmission with an OD in it ?? you don't need 15 gears, just a top end.

Read up on the Mack Triplex and Duplex transmissions. They kept you busy shifting gears, one had two gearshifts, the other three.

brownie box is just mounted in the frame just like a un married tranfer case is with a short drive shaft  

Been driving a Ford tandem dump with CAT engine and Allison - it has 2 speed rear ends coupled with it - decent power most times and not a lot of use of the 2 speed . 
 Now have 5 x 4 in my old White 6-200A engin trucks 6:38 rear ends - top RPM 3400
My GMC gas has RT613 direct and 7:34 rears - dang 427 engine needs to be up to 5200 or so RPM to go down highway at speed 
 About the best one gets with overdrive on secondary is 1.3 increase 

Adam... I have an auxiliary transmission that came out of a GMC twin-screw truck... Don't remember the exact model offhand, but I THINK it was a Spicer 7XXX... it has an overdrive, one direct, and two reductions... I THINK the OD was a 0.73 ratio.  It's physically too big and heavy for my Studebaker project (it's a 2-ton, and not long enough) so I'll probably hafta find a different rearend... something that's more like 3.93 and 5.08... in order to give me interstate highway speeds with the engine and transmission I'd like to run...  but if your twin-screw truck has room for big and heavy, PM me your email, and I'll go take pix, perhaps this is what you need.