rebuilding a 273 180 HP engine

pistons are the only oddity, everything else is a 318 (except the solid lifters, intake and intake bolt angles if it is prior to about 65 1/2, 66 should be OK 67 318 changed auto trans pilot hole in the crank for a larger convertor snout, youll need a bushing if you get a post 67 motor) Seals can be done "on the car" you just need a air compressor and a spark plug chuck. The air pressure keeps the valve closed while you take the spring and retainer off to get at the now desintigrated valve seals. If no compressor, you can stuff poly rope into the almost TDC piston spark plug hole and then gently rotate the crank so the rope is compressed under the valves, then you can use your rocker arm shaft as a fulcrum for a lever action spring compressor. The retainers come off with a little tap and the keepers can be retrived with a magnet (careful you dont drop them into the engine)
The new valve seals may come with a very small condom looking thing. That slips over the grooves of the valve so the seal slides over the grooves, not into them. While the spring is out, jiggle the valve. If there is more than just a hint of lateral movement, check the endplay wiht a mic and see if the heads need to be freshened up with new guides. A complete valve job runs about 150 if you bring the bare heads in, guides (check and see if they knurl them or bronze line them, knurling is not what you want) seats ground and valves faced. Tell them its a mechanical valvetrain and they might face the tops of the valves too, or elect not to becasue you already have adjustability on the rockers. They can reface the rockers too if the tips are galled. IF you are not interested in the nostalgic, but more of the driveability, a 318 is identical and can be had for next to nothing as a running motor and offers more torque.