While I have my engine out I looking into installing a air fuel gauge. Has anyone welded two bungs in the headers so you get both banks. Also is the a gauge that can display (switch) each bank?

Not on 814 engine, but for the 6 cylinder with a carb throat per cylinder (pair of triple zeniths) I've Put an o2 sensor bung on each exhaust at tube - so 6 in all. Hope to be able to ensure each cylinder has correct afr. And then further back in the system another two, one on each secondary pipe - as you are suggesting. I won't be using 6 or 8 o2 sensors simultaneously though. Too expensive (for me).

In due course I may use one of the close to engine bungs to run a exhaust gas temp probe.

Overkill probably. But easy to do while the exhaust system was being constructed.

Dave

Check out this one.
Innovate Motorsports Dual AFR gauge

I use their portable unit and have been happy with it.

I did not realize they are that pricey. The only logical placement for the bungs is on the collector on the muffler just aft of the connections for a dual system.

I just bought their MTXL-Plus AF gauge w/ 02 sensor. It's pretty easy to make sure both carbs are running similarly, so I'm happy with just the one mounted aft of the collector on the muffler.

http://www.innovatemotorsports.com/products/mtxl_plus.php

If you are getting one, make sure it is a wide band sensor. Narrow band sensors are only good for monitoring fuel economy and will not help much with tuning.