[johnsjmc]

With 300000 mi that original 305 has done OK I have seen them go 6-700,000 in taxis but usually much less . Your cylinder bores will have too much taper to just re-ring and the valve guides and cam will be shot. The cost to rebuild a 305 or a 350 are almost the same The 305 will get much better gas mileage but the 350 will produce more power for sure.
There aren,t many internal performance options on a 305.The small bore limits the valve sizes to max 1.88 in and 1.5 exhaust.(used in the HO and many 4 bbl 305 heads. The 305 engine doesn,t like hot cams. The 350 can use big valve heads and will tolerate much more cam.If torque is the goal then the small chamber 305 heads can be fitted with the 1.94 intake. That gives a small chamber and a bit more compression . The small port 305 heads will flow up to about 4500rpm on a 350 .That big an intake will overhang the bore on a 305.
If you need the power go for a stock 350 or a later 305 with a roller cam is another option probably about the same power ,little less torque. I wouldn,t bother rebuilding the engine you have unless correct serial number/date etc is an issue