283 can take 2.02intake 1.60exhaust valves the stock bore size is 3.875 and when you ad the valve sizes together they come out 3.62 which leaves you a 1/4 inch of clearance 0.255 so you'd be safe on that part some guys run them right to the max on race engines. you can also bore it 0.125 to make it a 4.00 bore a 302cid but its hard to find pistons for the 283 and the 302. but i know guys to cheat it by using 327 or 350 pistons and then have to find longer connecting rods cause of the differences in the pin hieght. but stock they all use 5.7 rods you can even talk to a old man machinist at your local machine shop and he could help you with what your looking to do and point you in the right direction. and your heads have to be redone for unleaded gas but they do make lead subsitute additive think about the guys who restore their cars to origonal. so you really do have some options but over all its your choice on what you want to do and how much you want to spend. but 350's are the most popular and little bit cheaper to build if you want to go that route. alot of people on this site will tell you to just build a 350 or 383 but its your car and its your choice what you want to do with it