Stock 1989 318 w/ 48k Miles - Rebuild Recommendations

What's it going in? It should be a hydraulic roller engine. Other than pulling the valve covers, intake, oil pan, front cover, and spraying everything out with diesel fuel and putting in a timing set, rear seal, and valve stem seals, I would leave it alone. Get a transmission and starter on it and do a compression test and document what each cylinder is doing . There's not enough gain in port matching a set of 302 heads to merit pulling them. If everything has been reasonably maintained and not caked in carbon and sludge inside, there's not enough wear in a 48,000 mile 318 to merit tearing everything down. Before you take off the aforementioned parts, block off the heater and lower radiator hoses on the water pump (if it's still there). Fill it with cooling system flush and water and let it set overnight. The next day, pull the coolant drain plugs on the side of the block and scrape out the sediment so it drains, check the freeze plugs with a screwdriver. After you get it stripped and everything clean and the soft parts changed out, put everything back together with fresh gaskets and ultra grey RTV. Set up a good distributor curve with 14-18 degrees initial timing and 35 total mechanical -advance. Do 1/4-20 tap and plug the smog holes. I recommend the Edelbrock Performer intake here, I've used on the very same engine with a Quadrajet and good distributor curve and it ran well. I would not even worry about the heads and valve springs until I was going to upgrade the cam.