I'll share a few comments based on running a similar car (Challenger, 496 EFI, 5 speed/20% OD, 3.23 rear) on the road and HSAX course a little bit.

In terms of stiffening, the frame connectors are a start. There is benefit in reinforcing and stiffening the K frame also. Firm feel did mine with gussets, plates in failure prone strut and LCA mounts and full welding. Torque boxes help and also believe plating the LCA's helps.

The 3 speed auto + GV + 3.23 should put the gear ratios about where you want them as long as your cam is not too wild (what cam?). The ~20% overdrive will be a "top speed" gear or close. For cruising you could handle more OD with the stroker or lower numerical axle ratio. For sustained hard running (road race, Silver State classic ect.) you will need a low slip converter and good cooler to keep transmission temps reasonable and keep the transmission alive.

It will be a seriously fast car. I would not personally want super fast steering in that application. The stock ratio with a smaller steering wheel would be my choice. Also the fast ratio arms limit header choice.

Brakes sound good. I run a Cobra based kit with 13" in front and 11.7" in the rear. I started with Bear front only and drums in the back. Not the best. Hard to balance and I burnt up the drums at a track day. I installed the Dr Diff rear disc kit and a huge improvement. With the brakes you choose, need to check piston area, especially on the front. The Cobra based kit has smaller piston area vs stock Mopar and needs more hydraulic pressure. I wound up changing to a 15/16" master + dual 8" booster to get my setup to 100%. Hydroboost is another option.

Good luck!