How fast should total timing be in on a 340 with 10-1 compression 500 lift cam

That is most affected by combustion chamber design. What heads? If Mopar what casting number?

with open chambered heads I would go with 35/with closed chambered with quench or magnums with or without quench I would go 31. I would set the initial first with the vac gauge method then shorten the slots to get the total then work with springs then work with the can (if used). the FBO $22 plate is by far the best/easiest/cheapest way to alter the mech slot amount to get the total (initial+slots) that you want.

I have the edelbrock aluminum heads. have been ported and a MSD distributor

closed chambered (I am assuming) & quench or no quench

As starting point, I'll suggest all in by 3000 RPM... and tune from there.

Not sure which MSD distributor you have. If you have a pro billet that uses a bushing and need something that limits below 18 which is the lowest msd provides. Some of the engines want more initial timing and less mechanical than the MSD parts will provide.

I have CNC machined limiter bushing available for much less than other providers generally do.

Auto or 4 speed car, converter if stall is known, duration at .050 of camshaft?

I would start with 32 degrees revved up over 5000 RPM and then go drive it and see how you like that, if it is good then try 34 and see if it is better or worse
The spark plug location in the head has a lot to do on how much total timing your motor needs, I have helped a lot of racers with SB Mopars that had tried a lot of timing and ran faster with less than 36 degrees total

yes closed and quench. its an auto with 3000 stall

I would set the initial with the VG method then shorten the slots to get 31 then mix & match springs staying a bit under the pinging point at WOT up thru the gears on your hottest/driest (most likely to ping) day. I'm assuming the OE spring with the elongated loop has already been tossed & if not toss it & another lighter one subbed in, this is a good start & go lighter or heavier as needed & the slope is linear no matter what combo is in there (with the elongated loop spring out of there). then adj the can if used, same thing, staying a bit under the pinging point in everyday driving under varying load/RPM conditions. Bottom line NO pinging is permissable (you want to stay fairly close to that line but never over it). & you dont want the advance to start any less than 1-200 RPM above what the hot in gear idle speed is which is what it will be at stoplight to stoplight. All this will get you very close for a good start.

Thank you very much Robert and cab. good info and a good place to start. understand it a little more now

It's an MSD distributor...

At WOT somewhere in the low 30's. At part throttle you could have 40's or even low 50's. You want a lot of timing when you are part throttle cruising down the freeway.