If you park your SVX with the drivers side wheels on a tall curb, and then lift the body up a few inches you would have enough room to slide under and see the two 10mm head bolts that hold the thermostat housing in place.
Get a big drain bucket from Advance Auto Parts, a stock thermostat and gasket from Subaru.
When the engine is cool,
remove the cap first,
undo the bolts,
get an antifreeze shower,
wash out your eyes,
change your clothes,
and install the new thermostat and gasket.
Refill saved coolant, reinstall cap, and Happy Motoring!
PS;
thermostat #21200AA072
gasket #21236AA010