Just about what 79 Jimmy said
A `165 ECM like you have for the MAF setup has something called a memcal which contains the prom, along with a couple of smaller IC chips which contain the limp mode stuff for the ECM (in the event of an ECM failure, these small IC chips will run your car in a limited, but driveable mode so you can literally limp home.)
When you switch from 305 to 350, you need to replace the entire memcal. If you just change the prom to a 350 prom, you'll still have the 305 limp mode, and since the limp mode is fairly limited (not quite so much with the `165 MAF cars as the `730 MAP cars) you don't want to even think about running your 350 on 305 limp mode stuff. Mostly cause of the timing, a 350 will ping very bad on 305 limp stuff if you go more than just a very slow idle.
Replacing the memcal is very easy. There are 350 memcals all over ebay, and thirdgen. To replace it, you just undo the two screws that hold the metal plate on the front of the ECM, pull it off, push the little tabbies on either side of the chip down, pull the old one out, and slip the new one in. They only go in one way, so no worries there
