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.

95 Montero SR
3.8 MIVEC, Advance headers, 2 1/2" exhaust, Magnaflow muffler, OME shocks & rear springs, 2" body lift, 3" tank lift, 4.90s, TRE front locker, factory rear locker, Roger Brown Rock Sliderz, 315/75r16 (35") tires, Sport big brakes