273 to 318 swap 65 dart vert

I've got a 65 Dart Vert with a 273 with 360 heads and a edlbrock performer 318/360 intake. 1406 600 cfm carb and a really lopy cam. It's the car in my profile picture. I bought the car and have no back ground info on the engine build. Ive got it running good but it seems to be all bark and no real bite. step on the gas and it makes lots of noise but doesn't really pull. I was thinking it's due to the stock 2 into 1 exhaust. I plan on running fenderwell headers to try and make it breath better. From what I was told the bottom end of the engine is stock. If after I add the headers it doesn't improve enough I was thinking I might build a 318 short block so I can use the 360 milled heads on the 273. What other things do I have to do to make the built 318 work? will it swap right in motor mount wise? What about the torque converter? There's a 904 tranny behind the motor. Any help would be very appreciated. Thanks