I think you're on the right track. 250 has the same bellhousing as a SBC/BBC so as long as you buy a 200r4 that bolts behind a 4.3L/SBC you will be good. I'm sure the 450 cfm carb will require some tuning, but should work. You will likely need to shorten the driveshaft and mod the shifter and crossmember. I thought someone made a shifter that works for the 200r4.