best carberator for the stock slant 6

good advice guys, and so on to the next novice question. Back in the 70's when you could buy leaded gas with a higher octane level these slant sixes ran just fine but now that we don't have the same quality of fuel does the guy doing the the engine rebuild typically bore out the heads for larger pistons to compensate for poorer fuel or does he keep it the same as it was back in the day?