Hey Andy. Based on the bolt ons you mention you probably have picked up an additional 15 to 20 over the factory 220.
You're working in the right direction getting more air in and a good long tube exhaust set-up. Next best jump would be (a mass air set-up with) a cam, intake, 70 - 75mm throttle body and basic head work. No real need to go to expensive alumnium heads, just work what you have.
Best power for the buck improvement is a 373 or 410 rear gear, and YES, the 410 is fine for the street.
It's pretty easy and affordable to get to the 250-275hp range, but starts to get expensive to look at 300+.
The good news is a 100,000 mile factory bottom end is good for 300+ HP without any changes other then rings and bearings.
Good luck...
Ron