273 motor question

Depends and several options/issues. Rebuilding the 273 is probably cheapest & easiest, unless "smoked" means "broken block". You can bolt heads thru ~2002 Magnum to your block. If your transmission is fine, you can simply add a ~$25 adapter ring to fit the torque converter snout to a later engine (68+ or so). Could also use a later converter if you also change the input shaft (if rebuilding tranny). Keeping the tranny keeps your cable shifter and driveshaft. You can get ball & trunion rebuild parts (I did) or a custom driveshaft w/ slider section like many Jeep Wrangler guys do.

If a 318 (5.2L) or 340, you can bolt-up your 65 oil pan to get the proper clearance dent for the steering linkage. For a 360 (or 5.9L), you will need to strategically dent the oil pan. If you use the tranny for a Magnum V-8, you must rework the car's tranny tunnel.