serenity03k64

So I've had my 89 pickup since last fall and it's been a great vehicle, started every time even in -35f weather and has done everything I've asked while playing around offroad until a couple weeks ago after I changed my oil and it started running like it was misfiring and is dumping gas out the tailpipe. I ran Seafoam to help clean it out and it smoked like a chimney. I saw a fair amount of smoke coming coming from the exit side of the manifold right at the gasket until I changed the oil. Since I've had it I've only replaced the MAF sensor, spark plugs, cap&rotor, filters, fluids, and tires. One of the previous owners welded a new bung for the O2 sensor and the old one is still in place. I pulled off the cat and (poorly) welded pipe in it's place back in the fall and it ran fine if not better (aside from the embarrassing sounding pinhole in the weld). I had the gasket set so I decided to set the valve lash while I replaced the valve cover gaskets after it started running hard with no effect. Since I've owned it it has had a very noticeable clattering noise from the motor (not sure if important) and hoped valve lash would help. While driving I also noticed the tach acting strangely as I accelerated. It would stay at about 2000rpm until I hit about 3500rpm and would usually do this while hesitating and running at it's worst. I thought it may be an ignition issue so I pulled all the plug wires one by one to no avail. Tried an ignitor from a junkyard with no results. I love my little yota but if I cant get it running normal soon then I may have to let her go. Any and all ideas I can afford I'm willing to try and will be greatly appreciated. I can take pictures and videos if you want. Also there is a vacuum hose on the sticker that I can't find that I'm not sure if is related but worth mentioning.