Does anyone make a frame repair piece for this area. Mine is eaten away from the battery on the top of the frame, all the way to the firewall. Do not want to remove the whole frame rail.

Charlie Osborne makes that entire rail and you could just cut out the section you need.  https://www.datsunzparts.com/services/frame-rails/

Those frame rails look open, C-shaped. Whats with that?

If you look at the way the metal meets. The frame rail is three sides of the box. The engine bay side makes up the fourth side, I believe

Patcon is correct.  The 'inner fender' (which you might see as being the engine compartment side wall) forms the fourth side of the box.  Thar inner fender panel has a horizontal flange along its bottom edge.  For its part, the 'frame rail' has a vertical flange along its top surface.  The frame rail is joined to the inner fender by two rows of spot welds:  the first along the horizontal flange and the second along the vertical flange. 

I agree that it can be a head-scratcher until you stop and stare at the details.

I guess it looked strange because my inner fender is roached too. Do they make that panel for a 280Z?

I dont know of anyone who makes the whole inner panel but I think the guy in the UK makes patch panels for that area. His name escapes me right now

The company is Auto Panel Solutions, located in Thirsk, Yorkshire, UK.  No website. They advertise only via Facebook.  Prices seem reasonable and the CZCC members (not many) who have done business with APS seem happy with their purchases.