What should total timing be set at on a mild 400 with open chamber heads. I have heard you can get by with more advance with these heads.

Well, all B/RB heads from 1968 on are open chamber.
Most bigblock Mopars with stock heads like 38 degrees total timing at 3000 rpm.

You may not know this, a combustion chamber that "takes" more advance is inefficient and costs power overall. Every degree before TDC that the fire starts, costs something. The piston rising against the expanding mixture is doing negative work.
Modern combustion chambers get complete combustion with 28 - 32 degrees total advance and the result is more power and fuel economy.
I feel funny calling a combustion chamber with active squish a "modern" chamber as it was developed and patented in the late 'teens or early '20s by Sir Harry Ricardo.
The open Mopar chamber with its lazy action was the result of lazy or ignorant engineering in an attempt to meet emission standards in the late '60s.
R.

I'd set the initial with the "vacuum guage method" then with that figure in hand set the slot length to get 36 total (vac adv capped for initial and total amounts) then play with the springs then hookup & play with the vac adv (if used) IN THAT ORDER. 38 may work OK depending on octane/scr/cam. Experiment & see what YOUR eng likes. EDIT other than the initial, for the subsequent 3 adjustments (actually 4 since the vac adv has 2 you can make) you want to stay just under the pinging point on each of em

I found that 34 degrees was the best for overall smooth performance. 38 worked OK, but I think it may have had some silent knock. Just play around with it and see. Going to depend on your particular engine and the gas you have locally.

Total timing of 36-38 is probably what it will like. Now what rpm in wants the total all in by will depend on your cam, compression ratio and fuel.

The only to know for sure is a dyno or take it to the track and test. I would not be surprised if it likes 42°.