my slant buildup

I would consider the 340 spring or the Comp 901 the absolute minimum for a valve spring for the slant 6. Even a stock build.
Summit sells Valve Spring Height Micrometers for around $50.00
With one of those you will be able to determine how good of a job the machine shop did getting the seats all the same height as well as be able to determine what valve springs match up to your head and cam.