Ken
I would give Skip Haney a call down in Fla. Yuo know Skip's waterpumps. Well he also makes check valve for the overflow for flatheads & y blocks.
I put his check valve on my 36 coupe and the temp never went over 180 degrees even in 90 plus degree weather. I think the check valve costs $24.00. If you need his phone no# contact me back.
Good luck
Bill