"You can swap the block right
"You can swap the block right out, but really unless it's a complete powerhead (including the computer, rails, etc etc) your 150 block would turn into a 135.
Also, Merc added a second temp sensor to the head in 2001 (one on each head vice one on a single side), so the newer heads will have an "extra hole" that would potentially have to be dealt with since the harness (and computer) from the 135 would not be equipped for the additional sensor. (that is if the 150 block has the heads)
I don't know off the top my head of any other modifications to the powerhead itself, but Merc was wrestling with cooling system issues on these, which resulted in the second sensor - just don't remember any other significant change in the 2001/2002 model year.
Alternately, you could use the heads from the 135 on the 150 block if they are undamaged.
So all depends on how much you are going to switch.
This being a 2.5L (152 cubic inch) block it's ideally suited for either 135 or 150 horsepower, so no real advantage one way or the other as far as horsepower goes - you just want to keep your horsepower equal to/less than the cubic inches to prolong engine life...
What caused the 135 to turn to junk in the first place? Has that been addressed - if not, simply swapping the block could result in a second pile of scrap aluminum...
I do have the service manual covering the 98-02 models and could email you selected chapters if you are interested/require them...."
