Ayuh,.... Welcome Aboard,..... It'll depend on how
Handy ya are, whatcha got for resources, 'n how much
Boat is in yer way,....
You can cap both sides of the y-pipe, or pull the motor, then the y-pipe, 'n cap the single hole in the transom assembly,.....
Ya gotta measure the ends of the risers to see what size pipes ya need,....
Then figure out the runs, 'n angles to get 'round all the junk in the boat to get out,....
I put thru-hulls on my barge 16 years ago, 'n spent 10 of those years tryin' to Quiet it down, startin' after the 1st hour of 'em,.....
'n I can tell ya,... the Only way to Quiet 'em down, 'n only at low speed, or sittin' at the dock,....
Is to get the tips 'bout 1/2 a foot,
Underwater,.....
If yer state has noise tests, you'll be payin' the tickets,......