Why not try rebuilding the existing transmission if it's not too bad. Iirc, Midwest Bayless in the U.S.can still supply some parts.
Swapping a transmission is not difficult. Very similar to a 500.
Disconnect battery (which is under a cover in the front luggage compartment)
Lift up rear seat, remove cover to get access to remove starter motor. Raise and support car. Support engine under oil pan. Disconnect clutch cable at release arm, speedometer cable, gear linkage, driveshafts at outer ends (at flexible couplings), bellhousing bolts/nuts?, remove transmission complete with driveshafts. You don't even have to drain the oil unless you intend to remove the driveshafts.
Only point to watch out for iirc is not to allow the driveshafts to pull out of the differential (final drive unit). The inner end of each shaft (under the rubber boot) has a crosspin and 2 drive blocks which can fall off. I used to run a length of cable over the top of the transmission and attach it to both driveshafts with hose clamps. Don't attach to the driveshaft flanges at their outer ends as these are on splines and can slide.
Variations on doing this job are to remove the engine and transmission as a unit by disconnecting everything on the engine, unbolt rear panel and pull engine/trans eother on a trolley or engine crane (cherry picker?). Also some people choose to remove the driveshafts before removing the trans. (just 3 or 4 small nuts holding flange over rubber boot) and refit the shafts after refitting the transmission. If removing the driveshafts, drain the oil first. Doing it this way allows you to ensure the little square blocks are still in place on the driveshaft crosspins.
AL.