I have the same boat.My tank(original)wasn't foamed in,but it was stuck on the bottom by their generous,(and sloppy) use of some putty that was used on the bulkhead behind the tank.
We ended up using a chain fall attached to the hold down brackets on the tank,to pop it loose.
My Tsunami is also an I/O and had a ford 302 with a early Alpha Mercruiser drive.I ended up buying an older Chris Craft Scorpion that had a nice low hour 5.7 350 Gm Mercruiser engine and a mid nineties Alpha drive, in very nice shape,for 1500.00.
I pulled the engine and drive,and gave the boat away.
Keep us posted with pictures,it reminds me of all that nasty,dirty,dusty,itchy,work that I and many others had to do to bring these beauties back to their glory.
*****PULLING THE TANK********
