d5n gearbox

Reminds me of a posting I read on a Yank Ford truck performance forum years ago when I first started tinkering with these boxes.
Bloke was complaining he couldn't do a 'racing change' between gears and wanted to know how he could speed up his changes between first and second and back again.
Couldn't keep up with his mates with their C4's and C6's!
Advice given? "Right there's your answer: pull the NP and fit an auto".

this might help

By using the extended mainshaft,you can mate these boxes to an early Land Cruiser transfer case.The reason I mention this is that some years ago Toyota dumped hundreds,if not thousands of brand new obsolete transfer cases here.Dunno if they sold them or scrapped them,but they were salvaged.$50 for a brand new FJ45 transfer(three speed box type).Still see them about sometimes,unused.Build yourself a 4x4 Dodge ute with a 318 in it.

Where would the handbrake Go?

Does your model have the handbrake drum on the back of the tailshaft extension?

Dodge with Spicer axle had duo servo brakes with handbrake in the drum same as a Falcon.

I'm not sure , as John suggests its in the rear brakes

When you asked "where would the handbrake go?", thought you may have had one of the models with the handbake mounted amidships as in the 475's or similar