ECM with new engine?

I've been told many times to do away with the weak little 305, and swich to at least a 350 or some kind of crate motor. my issue is if i do eventually decide to swap motors, i'd still like to be able to maintain the OBD computer and all. which tpe of engine would still be compatable with my type of computer setup?

If the engine is carbed with the E4ME, almost any small- or big block will work.

ok cool, so if i put in a 350 from a 9C1 do all the connections just fit right in the same?

Silicon would know better, but a 9C1 may be slightly different in terms of some connections and/or sensors. You're best off taking off all electronics and sensors from the 305 and putting them on the 350 including the whole carb. The computer won't know the difference, but you sure will.

that seems like it would be the best thing to do. i want something thats powerful but not a gas sucker too, i figure a 350 would be fair for both

Naturally a 350 will get less MPG than a 305, but it won't be all that bad. With more power, it will take less effort than the 305 for the 350 to get the car moving thus using less fuel, but being that theres more volume in the cylinders of a 350, it will naturally consume more fuel than a 305 overall. Assuming you drive in a not-so-agressive way most of the time, mileage will only decrease by a couple of MPGs. If you're always romping on it though, you'll more than likely notice a significant increase in how thirsty the engine can be.

With how slow a 305 was in my Caprice (in comparason to other, newer, lighter, faster cars on the road) I can't imagine how slow the 4.3L 6 cylinder must have been in the Caprice. I wonder why that engine seemed to be so commonly found in Caprice's towards the end of the boxy era.

My car was built with a 305 in it. I replaced that with a 350 back in '04. The trick is to get the 350 block and heads. Use the intake/carb/distributor/sensors etc. from the 305. You will have to adjust the air valves on the Q-jet, and you will likely want to rejet the carb for the increased fuel flow needed by the 350. In my case, the fuel econ went from 15/24 to 13/21. Not really THAT much, but the 350 makes the car fun to drive.

Don't worry about the ECM, or worry knowing that the 305 PROM has a more aggressive spark curve which will help the 350 run stronger than it would with the correct PROM.

I see, Pete brought up a good point too, i could only imagine how weak the 4.3 V6 models were.