I know that Competition Cams offers a "comp composite" box upper that will fit i think stock 5.0/5.8L style intake manifolds. I'm not sure if the runners are the same size between the stock 5.0L and 5.8L manifolds. You could probably contact them and ask.
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'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT