When I had my 78 Mainship I, I replaced both of the larger ports. I used Becksons from Defender, and they fit the hole exactly, however several of the holes did not line up. Just a minor issue.
I don't recall the part number but Newbern is probably correct if the sizes match up.
Getting the screws and barrel nuts out was a challenge.
Just remember that the 78 model year boats were built differently than the 79 and newer ones. There are quite a few differences.
Ex boats: 1983 40 Albin trunk cabin, 1978 Mainship 34 Model 1
New Port Richey, Fl