My Many Problems with my 88 sierra

[snatchmasterflex]

So I buy this truck last june and it was doing AWESOME up until a month and a half ago. I was driving away from my house when I noticed it revving 3000 RPM without even barely pressing the throttle, then bump and it shifted up into 3rd.... I lost 2nd gear. I am told this is kinda weird for a 700-R4 tranny cuz they either lose overdrive and eventually lose the rest of the gears or you completely just blow up all of a sudden. I pulled the tranny pan off of it and upon close inspection found very little shavings just a bit stuck to the magnet. I was wondering if anybody had any suggestions on wether to rebuild or buy a new one. Second Problem. My driver side fuel injector is runny extremely rich and when at idle it sounds like I have a huge cam in it, that and you can smell the gas very bad in the exhaust. I used to get 26 mpg to tha gallon which is good for the ol' 305 considering my old 85 scottsdale got 6 mpg, now my truck is struggling to get 7 or 8 mpg. I was wondering if I just need to buy a new injector or if there is anything to do to fix it. For the past 8 years the previous owners have been running it on propane so I think it may have just been gummed and and is starting to crap out. My third and final problem is that just mild driving around in the town I live in will send the engine temp soaring through the roof. This all happened after I got the intake gaskets changed. I also changed the thermostat awhile ago so that shouldn't be a problem I don't think. This recent problem has made highway driving near impossible seeing as the heat will redline if revs stay constant over 1500rpm. Also I don't know if it could affect the running temp but the heater core in my truck recently blew up so I took the hose that ran to the firewall and put it up to the hose on the back of the intake. also on the intake the "nipple" as GM calls it that is mounted on the intake was replaced the same time as the intake gaskets were. Also one more thing, I seem to have a problem with the vacuum in my truck. I was told it was the EGR valve so I replaced it but it didn't change a thing I was wondering if it could somehow be related to the poor idle my truck has due to the injector. Thanks for listening, any help anyone could give my would be greatly appreciated.