the problems you are talking about referr to the factory heads off of the L31 equipped trucks. These heads have large valve guides that limit lift to around .450-.480 and dont allow the use of dual springs. Also the 908 castings off of some box trucks have pressed in steel exhaust valve seats that limit flow.
These heads are excellent budget performance heads, i have a pair myself that i plan on using on a 450hp 302cid small block. The thing to look for is if youre getting your heads from a junkyard, get the ones that have casting numbers ending in 062, and use a dual pattern camshaft because the vortec intake port flows more air than its exhaust port, so youll need a cam that favors the exhaust. If you plan on running a high lift cam, with factory heads, comp cams has a beehive spring that will allow you to run higher lift, or you can have the valve guides machined down to allow the use of higher lift and dual springs.
The heads you are looking to order from SDPC have more than likely already had all of this done, but i would email their support staff to make doubble sure.
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