How much power can I draw from the fuse box

A vehicle that old and this make you just asking for a meltdown. You need to run dedicated circuits for all that with overload protection, good power relays, and noise suppression units. I would highly recommend you install another battery for this system with a continuous solenoid linked into the ignition and charging system.

Electrical systems for cars and for buildings are highly detailed things. Supply through fuse boxes is designed with little flexibility; if you have all the options in your car, there's really n room left in a box. With all the load of that inverter, you need to run an additional fuse box, dedicate circuits and fuse sizes, and hope the alternator can keep up with it when everything is running..... For better advice, see a car electronics or stereo shop. They'll assess loads and advise on what's needed to handle it.