Ive always thought the slant 6 was sick. I wanted a Dart when i was looking for my first car but went with the 65 Ranchero. The 200 is a great direct drop in engine. Just make sure you get a 65-67 so you have the small bell bolt pattern (66-67 are dual drilled, big and small pattern). That way yoy can keep the tranny and clutch you have and make it one less expenditure. If you're looking for a big upgrade you can do the T5 too. Or put in a C4. The C4 may be an auto buts its one of the best autos ever made. Plus it allows the use of the small bell so you could technically buy it now and get it installed (but its not as simple as slapping it in. You'll need a new tranny cross member, torque converter, l6 bell if its a V8 one, flexplate, possibly a new starter, new radiator with cooling lines, vacuum line from engine, and a kick down on the carb also with a dashpot, and driveline modifying, and new column). Its not super difficult, its just a lot of parts but doable for a beginnerish person. The driveline mod is something a specialized driveline shop will do, cost me $140 for a shorten/balance/V8 yoke.
Good luck,
Ryan