So anyhow I was looking round my local junk yard yesterday and i saw an old 48-54 chevy 3600, and it has the older 235 with the two holes on the front of the block for the water pump flow, BUT it has a 2 groove pulley on it which caught my eye. So the question is will it fit on a 58-62 chevy 261? Are the crank snout that they mount onto the same or are they totally different? Because if they are the same I can nab it for 10 bucks, if not no skin off mu nose right lol, any help here would be greatly appreciated, thanks in advance, Tye.

Crank snout should be the same. If it is worn a little larger you could use a commercially available sleeve.

so the diameter on the dual groove pulley is about a half of an inch shorter than my original, but the diameter of the heavy balancer part is the same diameter on both however. I also believe that the opening that goes on the crank is the same size. Should the difference of a 1/2 an inch smaller groove diameter make a difference on the pulley to accessory length like belt wise or is it not a big deal at all?

Being a half inch smaller it will slow any belt attached hubs down a little,like the water pump for instance. I would pay attention to the charge from your generator/alternator though just to make sure you're still getting enough juice for your battery and accessories. But, over all, I don't think it will have much negative effect.