IIRC, somebody on this forum was able to do the 3.5 DOHC valve stem seals without removing the heads, but when I did mine several years ago I tried 3 different valve spring compressors with no luck (really limited space around the valve springs on these heads) before I finally ended up pulling the heads off to replace them. I did everything on my motor at the same time - timing belt, valve stem seals, lower intake manifold, plugs, wires, belts, hoses, radiator rodded, even had a valve job done since the heads were off and getting disassembled anyway.