1990 F-150 Need Help desperatly

I am having to move from Iowa back to Texas a complicated situation with my Fiancee. In a rush I had to get a vehicle since we shared one. I bought a 1990 F-150 8 cylinder from an auction. The truck had sat 7 months before being auctioned. When I drove it it was missing a bit we thought that was the only problem and might be due to crappy gas sitting in the tank. Both tanks were almost empty. I filled them up and added injector cleaner based on advice from a local mechanic. The truck was not smoking at all when driving the 8 miles to the house. I even let it run some while checking it out. THis morning I ran it for about 15 minutes and it started smoking a bit. White smoke. I looked under and found the exhaust pipe missing halfway. It stops under the middle of the truck. I changed the oil from the advice of the mechanic and ran it a little longer and it started pouring white smoke then stops then goes again. The oil pressure was low untill I changed the oil then was halfway then after smoking dropped again. The mechanic I talked to says that it will do this untill the crap gets blown out but it seems to be getting worse and I am getting worried. Any help would be greatly appreciated. Could any of this have to do with timing also. I pulled one plug wire off and the truck idled as if nothing had changed. I will change plugs but dont want to risk more money if I bought a heap.

How many miles does it have on it? Does the smoking go away after the truck has been running for awhile?

It has alittle over 122,000 miles. It smokes a bit and if yougive it gas at all once there is smoke its gets heavy for a minute then goes away. It does not just keep piling up.

White smoke can be caused by a blowen head or intake manifold gasket and coolant being burned in the combustion chamber. Hopefully your mechanic is right and its just gummed up but I have my doubts. DO a compression check, also keep a eye on the oil to see if its turning grey or milky, if so you have coolant entering the combustion chamber. What condition are the plugs in? Maybe the tell tale story.

If I let it idle it will have no smoke then a spurt with some smoke then none.

Let us know what you find out. It may be nothing.

Just out of curiosity I switched to the rear tank. It is almost empty but I let it idle for about 4 minutes and almost no smoke came out. The smmoke that did only did so at first after switching tanks. When I revved the engine a bit came out but nothing like the front tank. Could the injector cleaner in the front tank be causing the smoke.

white smoke is burning coolant. chances are a bad intake or head gasket, not too expensive. I would go ahead and change the intake gaskets and go from there. Something else you can do is the leakdown test in the cooling system, that would let you know for sure.

I had a buddy of mine drive the truck around a bit while I followed him just in case and so I could watch the fireworks. It blew some smoke when he first gunned down on it and then after 2 miles of driving the smoke stopped. We drove for about 1 hour and then floored it again and no smoke. A faint bit came out. I guess it was just junk in there. But it still misses. When running about 35 or more its fine but wants to die when you stop and at about 25 miles per hour it wobbles bad at 65 plus it runs great. Any ideas.

After sitting all that time you could of had condensation in the fuel tank (smoke problem)
Wobbling at 25 maybe tires out of balance, Lug nuts not tight, or (although unlikely) flat spots in under inflated tires from sitting .

Well its got that problem of wanting to die when stopping and when accelerating it is smooth as can be but once it shifts it feels like its dragging almost missing or something. I changed the plugs but didnt help too much. When idling it fluctuates a bit at first almost dieing then staying normal for the first 10 seconds of starting.