another head question?!?!
 

I have 856 heads and I am looking at getting 853 heads. Is there any real difference between the two?

Will the 853 heads fit on my silverado?

does anyone know?

Sure they'll fit, if by "fit" you mean bolt on. Otherwise you're looking at a loss in power across all of your usable rev range from the loss in compression. 853's are just another set of LS1 heads with 66.66cc chambers compared to your current 61.15cc chamber 5.3L heads. You'll lose about 0.9 compression off your stock setup, which is more or less 10hp. The 853's don't flow better than the stock 5.3's until 5500 rpms and higher. They're basically the same head with bigger chambers and 2.00" intake valves. That's about it.

Why the need for a head swap anyways? Just curious..........

Came across a deal and could not turn it down. 100 bucks for both sides.
Could I have them ported and polished or do anything to make them better?

Sure. Ported, polished, smoothed out intake runners and combustion chambers. See if you can mill them down to 58-59cc's without ruining the heads.

I think there's better money spent with that $100 bucks in getting your stock heads reworked. Mill those down for higher compression and let the machine shop have at them to work their magic.

so LS1 heads are junk compared to the 5.3 heads for trucks?

You know anyone that wants some LS1 heads? lol.

There's a reason why so many on LS1tech want 5.3L heads. They get a bump in compression without milling, and after a bit of head work the 5.3L heads outflow the LS1's substantially, especially after the 1.89" valve is swapped out for a 2.00 or 2.02".