What year is your car?
I have found a Magnum engine to be a very good performer. There already a 9.0-1 engine that just needs to breath and given a cam, if actually needed, to fit the operating RPM desired.
I equipped mine with a RPM, 650 AVS, vacuum advance distributor & headers into 2-1/2 exhaust from Summit racing. An excellent inexpensive kit fit very well with minor trimming at the tail pipes. I will cam it later wit a little help from Hughesengines.com, who have trick retainers for stock Magnum heads allowing more lift than what the stock set up allows.
Just be aware that a stock Magnum has a short cam snout that doesn't have provisions for a fuel pump eccentric. Hughes engines also has a trick part for that. You can swap on LA front engine parts for a full conversion. An expense that I would guess you want to avoid. It is a relatively cheap conversion, but then again, it is money out of your pocket.
On what trams to use, stay with a 904. There engine specific if you did not know. Grab a B&M flex plate to use with the Magnum external balance to a neutral balance converter. Which is cheaper and easier to get than a external balanced converter. Install a shift kit and call it a day.
IMO, an inexpensive B&M floor shifter is an easy way out instead of messing around with the column shift mechanics.
On the rear end, a 8-1/4 is plenty strong, inexpensive to upgrade to a sure grip and readily available for much much less than a 8-3/4. The 8-1/4 is highly underrated. They will handle 400hp & 400lbs. Of torque.
Minus the rear end and engine purchase, the total cost is very reasonable. The results are very good. The only other expensive thing you may like/desire later is an O.D. Trans. &/or Edelbrock Magnum heads.