Mild Built 340 Timing/tuning help?

For the initial, this will put you in ballpark.
Distributor starting point for a curve
I'm going to guess something around 18* BTC at 850 rpm when you get it worked it in.

To determine the best for your combination, you will have to iternatively make changes and then readjust and check for max power at that idle.
Your goal is the lowest rpm it will idle with power.
Power is proven by putting it into gear with the least drop in rpm (automatic), a little more difficult to prove with manual transmission but same concept.
For a given initial this means adjusting the mix screws and and reducing the idle speed screw. Be sure the transfer slots stay in their operating range. (.020 to .035 transfer slot showing below the primary throttles). see this post for illustration and explanation Setting Throttle to Transfer slots

Then adjust the initial timing a couple degrees and go throughthe process again to see if you can improve the power or idle speed.

To do this ^^^^ There must be enough spring tension on the distributor weights that the timing isn't advancing at the idle speed you are working with. This was the one downside of the DirectConnection race distributors. They were already advancing at idle rpms. Back in the day, for drag racing, they were willing to accept that. The trade off was easy hot starting. (Races can not be won if you can't pull to the starting line)

Since you are not using vac advance, an advance curve similar to those used in the Direct Connection tach drive race distributors will be fine.
see these posts
Performance timing

and a Mallory distributor on a distributor machine showing why Direct Connection and others built in a long slow mechanical advance in the mid to high rpms/.
School Me On Distributor Performance