Lots of info here. Don't chase shadows. Your problem is in 3 maybe 4 things to check. Timing, temp, octane, maybe fuel/air ratio.
Fuel/air can be checked with quick plug pull or a finger in the tailpipe. Heavy black soot in plug or in pipe, too much fuel. Very clean white tips on the insulators is maybe too lean. If engine runs okay I doubt this is your issue.
Temp, if it runs normally between 180-190 this is perfect.
Octane, it is a 318 it will run on just about any 87 swill you can buy. Except if it is really carboned up. You can get some cylinder carbon cleaners from a auto parts store, just follow instructions on the can and it will not hurt FWIW.
Timing, you say it is correct but you do not know how to set it (search you tube for videos). I'm going to take a shot from the hip and say something is amiss with your marks or you are looking at it wrong. One solution is to check timing marks for accuracy, but that is involved. Go borrow, rent, or buy a vacuum guage and set the timing and idle mixture screws to the highest reading you can get again see you tube for videos. There is a 1/2" headed bolt on the passenger side of the distributor that holds the bracket down to hold dist, loosen this bolt a little so you can move dist around. Give this a try I'm going to guess someone bumped the timing advanced to get it to run better/smoother to sell to you. You will just have to correct it.
