Identifying mercury powerhead

[Gillh2o]

Hey guys. I'm new to here so first off hello.

My question is I have a 1979 175hp blackmax. I am changing out the center section for a 1993 model. Mine is 20" with 5" adapter. The 93 is 25". I believe the power head may have been replaced at some point. How can I identify if it has and with what year and/or size. The date on the side of the block behind the timing trigger is 6 15 8. Is that June 15, 2008? And the block casting number 858-6283.

My next question is which exhaust plate should I use? Should I do any mods while into the exhaust tube? I'm new to this and on a very limited budget.

I am curious if this is 2.0 or 2.4 and what will help it. I have a bayliner trophy 21 ft walkaround cuddy that we use for fishing and tubing. We have taken it to the beach once and ran it in the bay. It runs 39 by gps in saltwater with two ppl and 36 with two ppl in fresh. Thanks in advance for all your help and input