The 305 and the 350 are the same motor, just different cylinder bore; short-block anyways.
If you like flowmasters on the 3rd gens then you will love them on your Monte.
2.5" is fine for right now as the 350 is only putting out about 150-160hp.
Are you for certain it is a 350 and not a 400? Pros and cons to both.
How much do you want or can you spend right now?
Anyways, a full 2.5" dual system with 2 2chamber flowmasters sounded SWEET on my brothers Chevelle and my bbc Suburban. Actually you COULD go with 1 chambers, but I wouldn't, but they are loud!
Until you double your power output, I don't even think upgrading your pipes is needed, unless they are 1 7/8"
Next would be an intake, and I mean the metal one, as in the golden days an intake was metal and the "cai" of these days was known as an "air filter".
You can get a "chrome" one with a bigger filter, but that is not necessary either.
Edelbrock make a very nice Performer RPM intake that responds real well to MANY upgrades.
As you continue, you can either get their cam/intake/carb combination package or buy each seperately as time and money permit.
Header is also a good idea as you can remove those heavy ass manifolds and replace them with some inexpensive NICE long tube headers for under $200. These will adapt to your current exhaust fairly easy and inexpensively.
Get a Summit catalog and anything that will fit on a Camaro, Nova, Chevelle or their cousins will fit on a Monte Carlo; actually the Monte Carlo is a Chevelle/Malibu variant.
Rockauto is a good site as you can see SOME diagrams of stuff.