The head should have the same amount of material in that area as a head with a 1.74 valve and other than the open chamber it's the same as a 906 . I have a set of 906's with 1.81's , but I don't remember if I put seats in or not ? Call Dwayne Porter , he can tell you definitively , but the wild card is CORE SHIFT.

Does the shop have a sonic tester they could check the area ?