You can probably pick up a 305 or 307 cheaper than trying to upgrade the 283 and be ahead in many ways,you will have a large journal crank and rods which are stronger and a crank with a bolt on Harmonic balancer, a lot of 283's were pressed on.A 305 HO from a Camaro would be your best buy,there should be a fair amount out there for a good price since guys will swap for 350's and sell them cheap.