Definately check legacycentral for this. First of all, 91 Legacy LS's did not come with turbos. Did you add a turbo to your naturally-aspirated EJ22 engine? Or did you mean to say '91 Legacy SS / Sport Sedan...?
Anyway, the legacycentral board has complete info on replacing the timing belt and ensuring you've timed the vehicle correctly. Sounds to me like it could be timing at this point, since you've had to replace the water pump and mess with the belt, which could have jumped many teeth during your drive home.
Backfire during attempt at starting = clue that timing is off. Exhaust valve open at wrong time.