Since you want to keep the original engine, but would like 25-30 hp more, I would do this:
- have your factory lifters refaced
- engine should have a sol lifter cam if original. Going too big on the cam can easily turn a small engine like a 273 into a dog. I would use the Isky E-4 sol lifter cam. There are not a lot of sol lifters cams that have short-ish duration to chose from for that engine.
- have a multi angle valve job done on the valve seats. Have any sharp edges/ridges just below the valve seats smoothed/blended.