Not as Hipo as it sounds. What's up?

I've got a 273 with 360 heads and a edlbrock performer 318/360 intake. 1406 600 cfm carb and a really lopy cam ( no idea what the specs are I didn't build the motor). It's in the topless 65 dart in my profile picture. I bought the car and have no back ground info on the engine build other than verifying that the heads are 75 360 heads that have been milled to fit the 273 from the casting numbers. 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. What pistons are recommended? What years 318 short block do I wanna look for? What else is needed to make the built 318 work with the stock 65 904 tranny? also, can i use the cam outta the 273? And what pistons should I upgrade to to match the cam and intake? There's a 904 tranny behind the motor. Any help would be very appreciated. Thanks