NEVER use used lifters on a new cam and vice versa. it will fail. in minutes
lifters are supposed to have a slightly convex surface.(high in the middle)
most machines grind them with flat taper only the sunnen machine puts a proper radius on them

seat pressure isnt the issue as pressure only goes up as the valve opens
excessive open pressure wipes lobes
if the springs are coil binding you will wipe lobes
how much clearance do you have to coil bind? this is critical!

i would run 90 to 100 on the seat and 180 to 200 open
its only a little valve
too low pressures are just as bad as the valve train bounces. the lifter acts like a little hammer.

modern oils are designed for cars with catalytics converters.
what oil were you running?
zinc wrecks cat converters but is really good for flat tappet camshafts.
crane cams and joe gibbs do a running in additive that works.
with the right oil and additives you MAY have gotten away with 275lbs

does your "expert" actually have a valve spring tester or does he believe what they tell you in the catalogs?