Two problems.....

If they are cast manifolds, they are water-jacketed. You could machine
them, however, you would also need to manufacture a special sensor adapter
to thread into the jacket and seal. (difficult)

Second, the Holly system is not marinized or certified for marine use, that
I am aware of.
Don't let the Coast Guard, Sheriff, or DNR see it..........

Best bet would be to update your engine with later model Merc direct
injection.
"Michael Letchworth" wrote in message
ink.net...
I have a 1985 mercruiser 350 that I want to convert to Holley TBI
injection
and I was wondering if they make an exhaust manifold that has a place to
mount an oxygen sensor or can the original be machined for one?