Thank you Bill and Tom!  I will post a couple of photos of the removal just for posterity/archives.  I did cut the fiberglass bond down the port-forward 90 degree edge and simply bent the plywood away and down flat against the hull, leaving the front plywood piece in place.  With the three fittings disconnected this allowed me to shift the tank backwards to release the front (cracked) fitting, then towards port over the flattened plywood, then up and out.  I'll just use some metal strapping at 45 degrees across the top of that corner to stand the long piece back up and connect it to the forward piece.

Note that I've had considerable success 'welding' plastic tanks (in the last case it was the ruptured black water tank on the old Mirage 26) with a hot melt glue gun and extra strength (yellow coloured) glue sticks.  I actually repaired an 8" long rupture on the top of the waste tank (mud wasps had sealed the vent and pump outs had then put so much pressure on the heaving top of the tank that it split) and it held permanently afterwards (with regular checking of the vent to ensure it was not clogged again).  See attached photos.

What I will do is first hold the cracked spigot tightly closed with pliers and tighten a thick nylon zip tie around it to hold the crack closed, then slightly melt the crack above the zip tie closed, then build up a collar of extra strength yellow glue stick all around the spigot.  I have to do this on the top vent spigot as well.  With luck the 'weld' will hold permanently and will not require sealing over and installing a new fitting per Tom's suggestion.

I will drill a new large door handle sized hole on the center bottom of the long port-side plywood piece for ventilation/drying out as (I'll post a new photo) the inside of that side had some black mould from the tank overflowing through a crack on the vent spigot and around the 4" access hole cover.  I HATE black mould and generally do a thorough spraying under the floorboards from bow to stern every year or two with bleach or Moldex followed by a lot of water sprayed under everywhere to rinse things out and allow the bilge pump to prove it can handle the inundation.   

ANOTHER NOTE: as the overflow when filling the tank seems to be an issue for many owners, note that when the tank does overflow there are crevices behind this tank around the rudder post and down the center line below the water tank on the inside of the skeg that hold a good deal of stagnant water.  I dry these spots when I discover them, then fill them with expanding foam that I allow to dry, then cut level with a serrated knife and over-coat with waterproof paint/epoxy. 

I long ago gave up on the idea of a permanently dry bilge (our fridge hasn't been updated yet and we use a block of ice to 'help out' the Alder-Barbour 'freezer' unit that then gradually melts and runs down a hose I installed that replaces the old foot pump and runs into the bilge) and instead try to minimize any spot where water might accumulate.  This required a series of carefully Dremeled out holes in every cubby and cupboard against the hull to ensure they all drain properly if any water/condensation finds its way in (as is the case when I deluge the inside of the boat to rid it of black mould below the waterline!).

Kevin