Engine swap in a 96 chevy silverado need help Thanks?

I will answer this the best way I know. The engine and trans. are "dumb". They are told what to do by the ECM and act accordingly. I think with the 305 you probably have a 4L60E trans. I believe the converter should be the same for the 305 or 350. Check with your local parts supplier and they should be able to run the numbers and tell which is which. Did you reuse the same trans? I would say you either have one of three possible problems, either a problem with the trans, or a programming (ECM) problem, or you may just need the trans adaptive points reset (using a scan tool). The torque converter should not be an issue as lock-up doesn't occur till you get up higher in the gears and depending on driving conditions usually around 40mph at the earliest. Check your basics first, fuses, fluid level and condition. Recheck to ensure you hooked everything up(grounds). Now the good news if it is a computer issue (ie: recognition of 350 vs 305) you should be able to reprogram it with an aftermarket programmer instead of buying a new ECM. You cannot simply replace these ECM's without it being programmed to the vehicle anyways. It knows all your vehicles vital info (VIN number, etc.) Bad news is the programmers are kind of costly. If you can't narrow it down to the root cause seek help. It will be cheaper than burning up your trans and forking out funds to replace it. Your stock alt. should be either a 100 or 105 amp. Mine is the 100 amp.Good Luck! Hope I was some help.