Direct the heat at the screws - it will be the thermal shock that weakens the corrosion
bond. If directing heat at the aluminium be a little careful as it can melt, without warning, even from the heat of a primus torch.
Here's another couple of other ideas to help break them loose if you have access to the gear
** Put two nuts on the thread tightened together and use an impact wrench (rattle gun) to try and break the bolt free from the corrosion on the base (have the gun set in the tightening direction).
** Use a socket behind the heads of the screws that will support the base around the screw head
while allowing the screw itself to drop into the inside of the socket once loose. Give opposite end of the screw a rap with a hammer.
FYI I restored a corroded cast aluminium mast base on my previous boat that looked pretty much like yours. If I recall
even the bolts were an exercise to remove too. I power wire brushed it to clean it, filled all the pitting with epoxy
filler then fared, etch primed and painted. Came out looking brand new and stayed looking good for at least the few years I owned the boat.