What next..?

[Goose]

There was some missing from my 350 Chev engine on long trip. Did a compression check when I got home, and all cylinders except 6 & 8 are within 5 psi with an average of about 160. Except for 6 & 8 which came in at 120 each. Plugs for 6 & 8 were badly fouled as well. I obviously need to fix something but where to start? Will a simple valve job and new seals be enough with new head gaskets or is it more serious as in potentially new rings. Engine has approx 137,000 miles. A complete new long block seems most likely, but significantly more expensive than just a valve job if that would cure the ills.