finishing up my 65 dart GT 273 HP four speed

I'm still thinking about Installing the 340 in the car. A good friend of mine had to re-enforce his cross member, and I hate to alter anything. Will my 340 bell housing and flywheel fit ok? Also, the dart had only one exhaust outlet. any alternative to headers?
Everything else is done, disc brakes, 340 radiator, 8 3/4 Differential with 3:55 ratio,dual master cylinder, splined 68' four speed. new 68' 340, as well as a rebuilt 273. The 340 is bone stock, no racing,(sort of) I will be Installing vintage A/C when done, and Intend to travel with the little cutie! Any advice from someone here who has made the change would be greatly appreciated!