That answer depends upon how the motor was built. Originally the 283 and the 327 ran on regular grade gas. Problem is that leaded regular grade gas from 1967 had an octane rating of 94 which is one point more than high test is currently rated here in Florida. If the builder put together a late model 350 and used similar 72 cc to 76 cc late model heads then it will run all day on 87 octane unleaded gas. As the combustion chamber volume contracts then either you need a set of dished piston or higher octane gas. So if you see the original Fuelie heads (double hump casting marks without accessory bolt holes) then they are 64 cc chambered heads and with a flat top piston (stock four eye brow valve reliefs) will probably require the same 94 octane that it originally required. You can look on line for the casting mark identification of stock Chevy cast iron heads. That will tell you the chamber volume.
Your mechanic can run a simple compression test to see what your compression ratio is. You are looking for about 9.2:1 for the absolute maximum for premium pump gas; a lower ratio to have any hope of running 87 octane. You calculate the compression ratio by dividing the averaged measured gauge pressure for all eight cylinders by 14.7 to see the compression ratio expressed in atmospheres of pressure. You can run a higher compression ratio with aluminum heads because they allow more heat to escape the cylinder than a cast iron head does. From the ideal gas law we learned in ninth grade Chemistry we know that pressure (compression), volume (displacement), and temperature are all related.
Last edited by Big Dave; 07-27-2015 at 04:50 PM.
Reason: no I didn't KNOW that (wrong no got through splelng cheker