Looking at some photos of 250 chambers - some of the intakes look really big, but no dims given.
If the chamber is already bore size (3.875" + O/S etc.) the valves are probably past the point of diminishing returns, as the near edge of the valve is really close to the chamber wall. That side simply won't flow anything, and (more important) also biases the direction leaving the port. Unless volume is really critical (1" valve, 500" motor) this looks like a case where the largest effective valve is perhaps whatever OD clears the bore edge by at least 1/2 valve lift.
I don't remember seeing it explorer but the current philosophy (Jon Kaase) is that the intake valve should be:
1. as close to the center of the bore as possible
2. as large as possible
3. the exhaust valve gets whatever is left
This means moving the guide, rocker stud, etc. over and pushing both valves towards the exhaust side - example only: move the intake 1/8" and the exhaust 3/16". This may not be possible since moving the port walls hits water, but you get the idea.
