On your deal I would call Manton(951-245-6565) and order a set of series three 3/8x.080 with both 5/16 ball and cup
Those spring pressures don't need any thicker on your motor, no matter what others think or recommend. There is no sense in wasting money on parts that exceed your needs unless your planning on going to a solid roller cam with 250 lbs.+ on the seats and over 800 lbs.+ open pressures in the near future
I have used 7/16 x.140 single tapered Manton on a race motor with over 800 lbs pressure over the nose, Manton recommended 7/16 x.095 for that motor, I insisted and the made them and sent them to me
They where correct, I didn't need them
We can learn from others, if we listen
I have had more than one problem with Smith Bros. since moving up here from SO CA, they where less than 5 miles away from my shop. Now there 22 miles away in Redmond, OR.
Quality parts are worth the effort to get, especially when there the same price or lower including the shipping