Cheap Magnum Heads and some porting

With 160K on the heads, I would suspect they'll need guides at a minimum. So long as the seating surface is concentric, lapping can improve sealing somewhat at the expense of widening the contact area. Guide clearance is typically the killer with high mileage heads.

But, if you want to experiment, port away, lap the valves and bolt them back on. It'll only cost you a gasket set and time. At the end of the day you'll just go from high mile heads to ported high mile heads