my buddy has an 89 RS with the 305 tbi...
it starts and runs fine
the problem is that when you turn the key into the off position and even take the key out it will continue to run.
its not the ignition switch, we replaced it...so im stumped on it.

 

i dont know if this is your problem or not, but my 87 is actually wired to do that, when i swapped my 350 in, the terminal on the starter that was only suppose get power when the key is turned on was getting it all the time(this is where my distributor was connected), it as due to a bad relay but i couldnt find it, so i just wired a manual kill switch into the car, that shuts off power to the dist manually, if your dist is getting power then the engine will continue to run as long as its getting fuel. should give you a idea of where to start.