Yes, a little confused. I was only refering to D3 heads not the Dove heads. What is your opinion on valve sizes ? Is it necessary to increase their size as the motor gets bigger ? Or will the stock 2.08/1.65 valves work just fine as compared to the 2.19/1.71 in a big stroker ? My line of thinking is as the motor gets bigger it would need more cfm. At some point I'm going to stroke mine & if I could get away without putting bigger valves in that would be great.

 

I would at a min increase the exhaust side to 1.71 as the exhaust is the major restriction anyway, but even on a work engine I personally would go ahead and do both, I don't think the intake would really be necessary on a <5000rpm engine but I don't think it's going to hurt you in the least on a stroker. JMHO
could the stock sizes support a 521? on a low rpm truck engine I would say definitely maybe. With some clean up work yeah they would support it but if your replacing valves anyway why not go ahead with the slightly bigger. (once again personal opinioin but I think replacing the valves is a good idea just for safety sake, instead of living with 20+yr old 2 piece valves, decent stainless units from ferrerra really aren't that expensive anymore until you get into the race stuff of series 6000 and above)