The 91 318 has the 302 heads or a later casting number version. It has one of the smallest cams known to man. You are building a cruiser. Here's what I did with my '86 318, basically the same engine.
1. New rings and bearings. Very little wear visible after 115K miles, no ridge in the cylinders.
2. I swapped camshafts for a 360 roller replacement cam. Something like 200 degrees at 50 lift and .400 total lift. The 360 roller engine used the same springs, no need to change them. I actually did because I wanted to be able to run a larger cam down the road.
3. I home ported the heads. Steve Dulcich wrote a very good article about porting the 302s. If you cannot do the work yourself move up to Magnums, the 302s are not worth paying someone else to port.
4. I used the oil pan and pickup from the Poly motor because it fit the chassis and the block.
5. I bolted on a mid '70s 360 Thermoquad intake. Heavy but it works really well. For carb I used a Holley Street Avenger I had been using with the last engine.
6. I used exhaust manifolds that fit in the truck. I believe one was from a 1971 pickup and the other came on the '86 engine. I ran dual exhaust, 2.5".
7. High performance curve distributor with vacuum advance, I set total timing around 34 degrees.
8. I used the Mr. Gasket 1121G thin head gaskets.

Results: This engine increased mileage from 15 to 21 over the 318 Poly I had been driving, same vehicle. Also the distributor, exhaust system and carburetor were the same ones.

What happened bad: I had reused the '64 oil pump and it starved the engine for oil, took out the #1 rod bearing and burned the crank. DO use a new pump and DO check the pickup tube for cracks.

I was the world's third biggest Poly supporter and they're still pretty cool, but for a daily driver on a budget they'll kill you.

R.