Jake;

Refer to wiring diagram: http://sw-em.com/444LS%206V%20Wiring%20Diagram.gif

..."gauge flicks up (to full" suggests open between sender wire and chassis, and this includes sender (quite possibly open due to corrosion from extended nonmovement)...or just a full tank!!

Lights are not a fused load, so although cleaning fuses is a good general recommendation, it won't help with that...it may help with directional indicators...

Starter function is also not a fused load, so no amount of restoration of fuses will help there either...Derek has it right...PO MAY have installed a Start Switch (good idea!), and key no longer functions (or tears keys apart either) in the Start position...so you may want to take a close look under dash to locate this switch...no gray wire on terminal 50 (or ANY for that matter) of IGN switch confirms a Start Switch has been installed...follow that gray wire (which I expect leaves the harness in the area of the IGN sw, to the location of ST SW). To Start, use key to turn IGN ON, the momentarilly activate Starter by pushing Start Button...Viola!

Double check starter function by bridging input wire to battery power (when you allpy power, starter should engage and turn over engine).

I agree with advice to only feed fresh fuel, and empty and inspect what is in tank! Suggested reading: http://sw-em.com/Reviving%20a%20'decomissioned'%20Volvo.htm

Good Hunting