Probably a tee fitting between the engine and the sending unit would give you a port for the mechanical gauge, and keep the sending unit operable while running the mechanical gauge at the same time.
If that's not practical because of space limitations, you can buy a spacer called a sandwich adapter, that screws on to the engine and the oil filter then screws on to the adapter. It has threaded ports (pressure and return) that you can tap the mechanical gauge into. They usually come with pipe plugs in the ports so you can choose what port you need to use. The sandwich adapters are used mostly to plumb oil coolers or remote filters into the oil system, but if you were space limited and couldn't use a Tee fitting to tap into between the engine and sending unit, one of these adapters should work fine.
Here's a link to The sandwich adapters:
https://www.amazon.com/s/?ie=UTF8&ke...l_7jyovfn4uh_e
I have a Frantz by pass oil filter on my LS2 GTO and use one of the sandwich adapters to plumb it into the system.