Neo1874

Operation restore Cutlass is in full swing now (well the engine portion anyway). I was hoping someone could clairfy the valve specs for my heads. The casting number is 403859/ID code 6. From the specs I've found on this website and a few others the IN/EX valve's should measure 1.995/1.624 respectively, however the valves I have measure 1.88/1.56 which are the measurements for the #5 Olds ID code. No biggie I guess but I'm going to replace all the valves and lo and behold no one (summit, milodon etc...)offers valves with these specs. All that I have found are 2.07/1.71 which I figure are for BB heads. I guess my question is 2 fold. What size valves "should" I have and can my heads be machined (has anyone attempted) to accept the larger 2.07/1.71 valves?