The 901 is 1.650. That's pretty close. I would use offset retainers before I cut the head. JMO. I never like cutting the spring seats. On some heads, it's really thin in that area.
Like a slant?That's about .080-120 thick.
Some heads, like truck 318 heads, without longer valves or ...... you cant get a real spring in there. The passenger car heads seemed to have more room, then again a lot of machinists would just put the valve job down some.
Retainers are 150 bucks, but I know you meant what I said earlier with pop up locks.
But its really not a 'big wow' to cut .020 off the spring seat... that's how I know how thick the slants are
