Im wondering if anyone knows if I can use an aftermarket guage and sending unit and can it be used in place of the thermovaccum switch on the left front of the cylinder head?

I can't see why not. It will be placed into the coolant flow at the hottest part, right where it belongs.
Depending on what you can buy, you might need an adapter, or reducing bush, but as long as the hole is a standard metric size & thread, it should be a doddle.

also make sure the thermo probe does not obstruct the passage way, the passage wa there is not that deep.

Maybe if you can just get the tip into the coolant area it would be good.