Buying a whole car for the engine sounds excessive, but perhaps you could part it out and sell parts. I did that to fix our crashed 1985 M-B. A small-block will bolt to your K-frame. You need a V-8 steering link which drops down ~1" more to clear the oil pan. Some 1966 slants came with one. The drop is subtle, so stare closely at web images to see if yours is. I have a 1965 273 and 1964 225 so know both and have some parts. Indeed, too many parts after being too accumulative so slowly cleaning up, so PM if interested. In Sacramento and go to SF ~4 times per year, or you might pass thru going to Tahoe.
I have a 1965 273 engine I'll never use. It was a HiPo but pistons were seized and messed most up beating them out. The block# is same for Commando, HiPo, and regular 273, with only the pistons changing, so block has no extra value. The seller kept the special HiPo valve covers (fins). Would have to bore & hone the block for new pistons. I recall you can buy new HiPo pistons (Egge?). You will need a matching transmission w/ slip-yoke output. Transmissions vary between slant, small-block, and big-block. The engine and transmission must also match in years since ~1968 the torque converter snout got bigger (many posts). A 904 is best and sufficient. A 727 requires beating the pinch-weld on the tunnel flat to fit, and would give less mileage. I have a V-8 steering link I'll probably never use since plan to keep the slant in my 1964.
My thought for a slant upgrade is to wait for better motorized compressors. Much simpler install than a turbocharger or belt-driven supercharger. There are some already, though sketchy Chinese. The >$500 ones actually increase power (youtubes). I suspect they will improve with changes to maybe 48 VDC (w/ inverter), better batteries, and such. Boost overcomes the restricted flow in a slant head, as evidenced by this 9.09 sec quarter mile run. Slants draw a bigger crowd at car shows than the multiple small-block Chevy's w/ same Edelbrock bling.
If I swapped to a V-8, I would consider a 5.2L Magnum for MPFI. Parts interchange like heads and can bolt to a later 904 transmission. An early-A 273 oil pan bolts up, or you can dent the Magnum pan to clear the steering linkage (lateral extreme). Better is the Magnum auto transmission (overdrive, lockup), but must cut and reform the tranny tunnel to fit. The 5.9L is a 360 so requires those changes. Since 5.2L moved heavy pickups, it is ample for a light early-A. A whole forum on Magnum swaps. I would keep the single exhaust since even the HiPo's were single, albeit w/ special exhaust (can buy repo's). If you go dual exhaust, you must rework the transmission cross-over. One exhaust manufacturer offered that mod if you buy their duals. Coolest would be popping the hood to see a 3.8L V-6 or 2.4L DOHC (both RWD in Jeeps).
