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2.9l heads from '86 to '88 have a weak spot in the design/casting
'89 to '92 heads are a newer design without the weak spot, they often have 89TM on the casting.
3rd party heads are almost always the newer designs, and will say "for '89 to '92 2.9l"
The newer designed heads work fine on '86 to '88 blocks
The 2.9l was last made in 1992 so no real market anymore for mass production of replacement heads.
So you will need to look around, like on ebay, to find them.
Most sites I checked said "no longer available" or "discontinued"