I've got a set of prc stage 3 243s with the 2.08/1.60 valves, will they clear on a stock bore 5.3?

 

They will, but your compression will drop some due to the larger combustion chambers, unless they've been milled.

 

Those heads are designed for a 4.03" bore. In a 5.3 application the valves would be severely shrouded. Also, I haven't checked myself, but I would question whether or not the valve would even clear the bore. I would choose a different head myself.

Lonnie

 

I always heard that a 2.02-2.04 is the MAX you can safely put on a 3.898 bore. On a significantly smaller 5.3 bore there's not way a 2.08 would fit.

 

3.78-(2.08+1.6)=0.100.....TERRIBLE
Some 5.3 blocks can be bored out to 3.898-3.905 range; however MOST engine guys prefer to not go any larger than 2.04-2.055 sized intake valves even on the LS1 sized bore.
Different heads is probably your best option in all honesty.