I have a Richmond 5-speed I built for racing and I currently run an 833 street HEMI 4-speed for just street driving. I have built a few overdrive 18 spline boxes with Jamie Passon's overdrive gear sets and they turned out good. I think you are smart to want mechanical or external clutch operation. I would HATE to have a hydraulic release bearing fail because you need to disassemble everything to replace it.

I have been inside a few Viper T56 transmissions they seem plenty strong enough but they are not known for their power shifting.

I like the Richmond for the 3.27 low gear and 1-1 final drive that way you can run a 3.23 gear in the diff for good road manners at 70 MPH.

Just make sure you spend some time with your fabrication the first time and make sure your driveline angles are correct.

Gus