273 or 225?
I see so kinda the same issue, another solution I can think of (although might sound stupid), is to cut and swap the ends of the driveshafts so that way the 66’ shaft could accommodate the Ball and Trunnion tranny. Would that be possible?
All 727's have large converter hubs, so it's a no on '66 & earlier 273's & '67 & earlier Slantys, not to mention the 727 is longer & takes a larger slip-yoke, so that's out.
Living in the salt belt as I do, the engine bay shot posted looks great compared to the junk out here, inner fenders/fenders full of holes all the time...You West Coast Guys are F'n spoiled.
For $1,000, I'd grab it, rear end may have better gear ratio...You could use or sell them.
The trans is biggest issue, but I know CharrlieS had/has converters for the larger/later input shaft made with the smaller early crank hub, so any later SB trans is easy-peasy..I don't know if they will still make them, but maybe Charlie has one He'd part with.
Honestly, as much as I love the Slanty, I'd grab that because the V-8 mounts, pedal/cable assy., centerlink, etc., all hard to come by, V-8 Wagon may have 10" brakes as well,...yeah, they're better than 9" binders so I'd swap them too, and disc conversions are available for 10" knuckles, not 9" AFAIK.
Also, don't worry about the "intake issue", yes it's unique with the vertical intake bolts...but it will run well w/a 2bbl, a little cam, a good timing curve, and a free flowing exhaust so...don't be skeert...& if You decide to do more than cruise, You can drop any later/larger SB in there & go back to a normal large hub converter in the stall of Your choice. Whatever You do, have fun & enjoy Your ride, peace!!!