Stoney48

I bought a single cab 93 chevy Silverado with a blown up 5.7. I bought a used 90 5.7 and put it in the truck. For the first month or so the engine ran fine. Drove it to a friends one morning and got ready to leave and the truck wouldn't start. Finally got it to run and it would barely move. Once up to speed the trans. wouldn't shift unless I made it shift. From that day until now the truck has been flooding and I only have 1st and 2nd and rev. with trans. Just a few nights ago it started bogging down and backfiring through the tbi. So far it only bogs and backfires from an idle to gunning it sitting still. I have changed the map, the coolent module in the intake ( yes I know the difference) rebuilt the tbi injectors and regulator and fuel filter. Oh and there is a loud hissing coming from the injectors. Sorry for the long story. Thanks for any advice