Hi SinWin,
An electric heat gun (for paint removal) will work fine.
Make sure to mainly heat the propshaft (stuffing tube) and not the surrounding hull, as the hull is much thinner than the glued joint and does require not a lot of heating up to deform.
I assume the hull is polyester, so once you deform it by heat the damage is irreversable, so keep an eye on it.
450 mm shaft is not a lot for the boat size, I have a 450 mm shaft in my 80 cm speedboat...
But a lot depends on the place where the shaft exits the hull and the hullshape in the aft section.
I would take out the woodblock completely and make a motormount out of 3 mm aluminum sheet.
I added a picture of what I mean;
The one on the left is a motormount that is shaped to fit into a single motor set-up, the one on the right is more universal and can be bolted into the boat on a bulkhead.
Notice the cooling slots for the motor, these are not to be blocked by the mount, as the fan inside the 700bb need to be able to draw air through the motor.
The motor should sit om the bottom with just a mm of space, no more.
The same goes for the prop, 2 mm cleareance of the hull is sufficient
Regards, Jan.