I didn't ghetto grind as the I have the guide tool. But have done the z28 springs with old school slower ramp up to about .450-.460 lift HFT's several times, about all they're good for. Not sure I would do it with a more aggressive roller cam. Typically z28's are 110-114 lbs. on the seat whether that's enough depends on the cam and spring specs. The heads i did needed the dampers pulled for the z28's also so it's all wrong for something you want to twist up.
FWIW
OE springs in Vortec heads (including 350/357 CPP crate) ... when new are about 80 lbs on seat aka closed. That 350/357 is a rather sporty roller-lifter piece; in spite of fact seat pressure ~ 80 psi.
350/357 valve lift is speced by GM at 0.473" both I&E. It's same spring PN that's OE in OE iron Vortec production head. Easily good to 6K in that motor with that hydraulic roller cam
Yes, like the OE Vortec springs (which have no dampers), one must delete the Z28 springs' flat dampers in order for spring to fit & hug Vortec guide bosses. Simply pull em out & trash em.
Also, during GM assembly, Many times those OE Vortec VS Seals are Not properly seated down onto guides; that further and unnecessarily restricts-limits valve travel before bottom of retainer smacks seal. There's an illustration and a spec for that fitment in GM Powertrain manual for L30 & L31 Vortec ... I don't have spec handy but it's NOT intended to push seal down until it can't move any farther; that'd be wrong ! ... so, please defer to GM spec.
*** Here's that spec in another man's post
So looking at replacing my valve stem seals and notice a couple big differences between FelPro (SS72861) and GM Genuine (10212810,12564852) seals. First is that the GM Genuine exhaust and intake are separate product numbers and look different, while the FelPro set supposedly has both but they...
www.gmt400.com
Ghetto grind retainer material about 0.090" away from bottom of retainer ... No problem so long as locks aka keys are Not proud of bottom of retainer. Dry-fit valve stem, retainer & locks on bench to Confirm locks not protruding below retainer.