[AcampoDave]

I would change the rear axle while making my transmission choice first. Having the right gears for your driving style would go a long way to improving the driving experience. Then, no matter what engine you go with you'll already have the gearing done. I wouldn't worry about rear disc either. I think those big ol' 3/4 ton drums would be more than sufficient.
In my own experience, a rear end change and a tranny change really took things to the next level. I spent a good deal of time online researching the available gear spreads of the tranny I wanted and used available calculators to figure the starting line (also called "launch") ratio of the first gear as it changed with various rear axle combos.

A quick version without figuring tire diameter into the equation is....1st gear ratio multiplied by rear end gear. For example, my truck has a 2.98 first gear and a 3.08 rear which gives me a starting line ratio of 9.17. As the SLR gets lower the engine needs to grunt harder to launch. Getting the SLR Into the 8's requires a bit more clutch slip (or engine torque) and getting into the 10's becomes more of quick wind up and a shift after a short distance.
Automatic trannys are a bit more flexible due to the torque converter and the gearing for those are all available online too.

As a rear end becomes numerically higher, the individual gear ratios in a tranny can be closer together whereas a numerically low rear end works better with a wider spread because you need to "stretch" things out a bit longer going from a stop to full speed since the tranny is starting from a lower (numerically higher) first gear. Final drive rpm calculators are all over the internet and overdrive would require one more calculation to come up with the final drive rpm after the reduction from 1to1 but I am not up to speed on that since my tranny is a non OD.

Nailing this can make any well tuned engine fun to drive.