I think with the 305 engine (even with mild performance mods) I doubt that you will be making more than 300HP. The larger the exhaust pipe diameter - the more CFM the exhaust pipe can flow. See
https://www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/
Based on the above link it appears that a 2" diameter DUAL exhaust would handle the 305 engine horsepower fully.
See attached excerpt on exhaust diameter versus horsepower from the above linked website. If you are going to install a whole new exhaust system for the LS motor (a waste in my opinion) then go with the 2" for the 305. And, then convert to 2 1/4" or 2 1/2" when you have the LS motor installed. According to calculated CFM/HP, the 2 1/4" should be good to about 370HP. While the 2 1/2" exhaust tubing should be good to about 460HP. These HP figures assume true duals for the calculation.
For the cruise around town 305 with 300HP or less, I would use 2" pipe with the Walker 21598 mufflers (originally found on late 70s/early 80s Z28s and Trans Ams. These mufflers are relatively cheap (about $34/each on Rock Auto). I have these on my 85EC with a 305 (nice sound and note but not too loud on the highway). If you go the 2" route than you should be able to use the Monte SS tailpipes (Walker 45910 and Walker 45909 for L and R tailpipes) about $28/each on RockAuto. You would then need to have custom bent pipes from manifolds/headers to mufflers.
If it were me, I would install 2 1/4" pipe and mufflers and then have the pipes all custom bent. Remember you can order the 2" tailpipes and have the exhaust bender copy them over in 2 1/4" pipe. This way you can likely use the 2 1/4" system when you convert to LS power.
Just my two cents.
PS Many moons ago I used to work in a couple different muffler shops bending custom exhaust all day.