Sounds like you have a Merc Marine 300 block. Relatively common.
You might want to double check on the hydraulic lifter oiling though. All the marine blocks I've seen are mechanical lifter only.
See if the block has had the two cross-over passages in the lifter valley (3/4 back on the main
oil spine in the lifter valley) drilled out and tapped for plugs. Or, for that matter, are there any holes in the lifter bores? $6200 is really pushing it on such a mix-match pile of parts still needing to be inspected and assemebled. To each his own and Caveat Emptor.
HTH,
Mike