Just got a call from the machine shop with a little worry about installed height and seat pressure. I'm using the enginebuilder 340 replacement spring and shop is reading 110psi on the seat and not enough room for std umbrella seals.
After a call to EPN about the springs he suggest shooting for 100 on the seat and 240 min open at max lift and cutting seat pocket as needed.
Looking for any opinions here, I thought this spring would be a fairly close drop-in solution but what do I know?

Right now we're thinking of machining the guides down for retainer clearance and running a pc type seal and the main concern is for cutting depth on the valve springs pockets, don't want to hit water . The band sawed head cut-away photos on the forum here make it look like there's a lot of room under the valve spring pockets, but my machinist might have to use a larger cutter, he doesn't think he has a small enough cutter on hand.

Any opinion?
Forgot to ask - It just dawned on me - Is the 340 valve spring retainer mandatory for these springs, like does it have a different profile the could fix some of these issues???

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1966 Coronet Deluxe

110 lbs on the seat is darn close to 100 and they will lose a little tension right away. That being said, don't cut the spring pockets. The springs should fit fine in the existing pockets. You will need to shorten the guides to accommodate increased valve lift. You'll also need 340 retainers to properly cover the tops of the springs. I've posted pictures on what slant six and 340-size retainers look like on 340-size springs.

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Joshua