I got a set of heads that are cut for 2.02 /1.60 valves. I am wondering if putting a 1.625 exhaust valve in instead would yield any benefit? would I need to have more work done to make use of the bigger valves?

Also what length should I get for the valves? I plan to run 1.6rr and have just under .580 lift on the cam (w/ 1.6). I will be running comp 918 springs and be running around .045 quench or a little less so valve to piston contact would want to be avoided.

 

I would get a .050 longer valve with your setup. This will make getting the correct valve spring height easier.

 

What's the point of a bigger valve if there is no modification to the seat. A bigger obstacle doesn't mean more airflow. The typical rule is to keep the exhaust around 80 - 85% of the intake size for optimal flow. They are not set that way at the factory but can be modified.