IF the car hasn't gotten the maintenance it needs, it could drop a timing chain or have an injector leak fire anytime. The fuel injection has a poor reputation for reliability: MAP sensors fail, the temperature sensors quit, the idle circuit runs away, and aging fuel pumps can take out the ECU. Cures for all this have been engineered if you want to deal with it. Some people just change to a carb.
The 350 engine will get you there, but it doesn't compete with 472 cars. A 403 will bolt in including the injection, using a 425 Cad ECU. Add switch pitch to your THM400 and good duals, and you are sure to take a couple seconds off 0-60. Bruce Roe, EFI 403 '79.