what size carb to use ??

I've got a 340 that was built for racing as a spare motor, what I was told about the motor when I bought it
1 11.5 to 1 compression pistons (Bill miller light weight forged pistons)
2 aluminum rods with a kellogg crank
3 hv oil pump
4 crane cam with solid lift ang gold rockers 586 lift is as much as I can find out dont know what the duration is
5 M1 single plane intake
My question is I'm running a holley 650 double pumper and 12.6in the 1/4 which I think the carb is way under sized for this motor what size carb can I run a 750,770,850,1000 cfm ??  

You didn't tell us what heads you are running and if it is in an A-body or B-body. If it is in the later the times are not that far off of what they should be.
However the carb is small.
How big is safe, a 750dp. Anymore without knowing the duration and heads info is likely a mistake.
Looking at what you have in the motor I would suggest 500HP is likely. A 750 DP annular discharge is good up to about 540hp so you can see it should be plenty.
In fact a stock M-1 when used with a similar 360 with iron heads ported and polished, developed 540 HP when tested by hughes.
Porting and polishing the intake is worth another 10hp if your heads can use the extra flow.
It would be good to know the gear, tire size and converter size to make a more accurate suggestion. Good Luck
Doc  

it has iron heads ported and polished both intake and heads
456 gear in the 8 3/4 posi with 26" 8.5x15 M/T slicks
3800 stall converter
in a 74 dart A-body  

Does it hook and what are your 60 ft times? If you answer yes and your 60 ft times are in the low 1.6 range, I would suggest a max. carb of 830 DP annualar discharge.
Truth is I doubt you are getting enough traction to get a great 60 ft. time with those small tires.
I run a milder combination and run 12.5 in a B-Body with a 28x10 slick 4.30 gear, 4000 stall converter and I sometimes have trouble getting off the line with a 1.6 60 ft.
I run a 750 DP.
Doc  

My 60 ft times range from 1.5 to 1.8 sec.
I haven't tried to fit a ten inch tire I think it will be very tight or not fit under the wheel wells are small on the a-body
I have been told by someone with a 318 with 340 heads and 750 cfm 410 gear he can get 11.4 in the quater ?? is this possible ??  

Yes, in an A body. I know of several guys running 360's getting these times in B bodies.
Its all in the combination. The #1 thing I see with your ride is tires. I can run 1.6's everytime I go out. So look into solving the tire issue before you do anything else. I see plenty of Dusters running 28x10" tires. You may have to upgrade the rims to get the proper offset to allow you to get the right tire on the car. If you can't get a 10" tire on then go taller, like 29x9 and get a D-5 compound from Goodyear if possible. Sticker rubber, better grip.    
Cam duration is important.
You have 11-1 compression so this will give good cyl pressure allowing for a longer duration cam.
Everyone I know is running a dual pattern solid or roller cam, eg. 242 intake @ .050,
252 exhaust @ .50.
Total lift does not need to exceed .550 lift with iron heads.
Frankly LA iron heads can actually run worse when the lift exceeds the .550 number, hows that you ask. The heads will begin to litterly jam up, they simply will not flow properly over this lift.    
So instead of going with huge lifts concentrate on getting the right duration for your set up.
Good Luck
Doc  

Thank you for you time you gave me some great info to try out
Terry