oversize valves

I would like to know if there would be any benificial gains in flow in the exhaust port by using a 1.88 valve instead of a 1.81 valve on a 440 dragrace engine the cam is a268 @ .050 .600 lift lightweight car with a5000 stall

It is difficult to say as a blanket statement. The person that would be the best person to ask would be the flow bench guy at your favouite machine shop after he has flowed your heads. Everything must be done in the context of YOUR heads. If you have had some port work or not, or had the bowl area behind the valve opened up, these things will all affect flow.
However,the most important thing is not necessarily the maximum flow number but the relationship between intake flow and exhaust flow measured at every .100 lift. As long as you have enough flow to make the cam work I have read the best relationship between intake and exhaust flow is 75%(best case theoretical)
To make a long story boring, will using a 1.88 exhaust valve over a 1.81 valve give more flow? If the port is opened up properly to accept the larger valve, YES. Do you need the extra flow on the exhaust side? DUNNO,maybe.

Just found an article in Mopar Muscle about BBD head porting. Looks pretty good. Febuary 1999 issue. Good luck!