Need Guidance on couple of 340 issues

Yes, Got a good look down there today. Copper tube coming out going to an aftermarket gauge....How did I miss that?!? Always thought it was a tach that was added. Not sure what I'm going to do. Not experienced enough to remove distributor with any confidence yet. I will put the dash bezel in next week and fire it up finally with key. Been working on rewiring dash harness and I want to make sure everything works before I tackle this. Don't think I'll need a mechanical oil pressure gauge but I will wait and see. It's been 5 years since this car started with a key. Thank you all for the help, you think you know a lot and then you brain lock on something like this.
Put a little Teflon tape on the male end of the sending unit, it could get a little messy if not. Sounds like you had a big project to start with. Before turning the key, double check all your hook-ups and make sure all the grounds are attached, and all connections are sealed from touching anything that should not be touched.