Chip,the valve to piston clearance wasn't a problem on a 235 and a 261 I assembled with after market cams The 235 had a Clifford 260 cam and the 261, the M4F. the 235 had .030 saved, and the 261 had 20 off the head and 30 or so off the block.At maximum overlap, both engines have .070 valve to piston clearance.If your engines pistons have close to zero deck clearance, its best to check out the clearance.The 40 thousands head shave may alter geometry slightly , but with the valve assembled to the correct height,you won't notice anything.
I use single springs, Buick I think, they work ok for 4500 rpm.When I bought the Clifford cam years ago, they recomended dual springs. The springs and retainers sold to me were wrong!! they didn't seat properly on the head (too small diameter) in fact their own springs didn't seat in their retainers.. Many phone calls to Clifford and they wouldn't admit to the problem.I replaced then on my own and tossed the Clifford retainers and springs in the scrap barrel.