For the V8 engines, you can buy offset valve keepers to raise or lower the spring hight like these,
http://www.classicenginesupplies.com/assets/classic-keeper-flyer.pdf. They may also fit the GMC, you would just need to know which valve stem configuration you have.
The SPI springs increase spring pressure by approx. 14.25 lbs for every .050" increase of compression. All you need is about .040" longer installed hight to make the installed hight pressure match. Since this isn't a race engine, I would lean towards the weaker side to save wear and tear on the cam and lifters.
Spring hight checkers are easy to use and getting cheaper, something like this,
https://www.ebay.com/itm/4929-Valve...-600-2-200-Most-V8-Measurer/124302909410SPI springs are likely for the newer inline sixes with slightly more aggressive cams, they just happen to fit the older engines with not quite the correct spring pressures.
Spring pressure, poor machining practices, cheaper material, and incorrect oil all can kill a camshaft, so you are right to be concerned about it. If you changed camshafts or lifters, ask the dealer about it.
Check the hight of each valve stem before getting too worked up, you may find there is enough clearance to set them where needed for the pressure you want. It's not all that hard for a shop to cut the spring pockets on the head just a touch deeper if you find that's what is needed, with the correct tool, it can be done as a DIY project.
Joe
