273 to 318 swap 65 dart vert

Isn't 360 heads on a 273 a bad idea? Doesn't that lower compression a lot? If you don't get much torque on the low end, that wouldn't be a problem with the exhaust unless there is a major restriction. I have seen mufflers clogged up. A vacuum gage can indicate that problem (slow rise after throttle), or if you disconnect it try blowing thru it with a hair dryer. Otherwise, it could be a bad O/F ratio, low compression, weak spark, ....

Horsepower is what you get at high rpm with the engine screaming like a banshee. It doesn't get you quick off a stop sign, unless you rev the engine with your foot on the brake as drag racers do (wouldn't that look stupid?). Most drivers notice low-end torque more, which is why most V-8's came with 2 barrel carbs.