What master cylinder can I use for disc on front drum on back? Drag car only. Converted from drum to disc on front now it’s real hard to push. It’s a 64 dodge 330. Thanks

What kind of discs up front? What MC do you have now? If the MC still has the residual pressure valve(s) it can cause the disc brakes to drag.

A Mopar tandem disc brake MC from a '67-'78 will bolt right up to the '64 firewall.

guessing it is manual brakes?

I ran into that issue and you need to look for the smallest diameter master. 15/16th usually.
modern with the adapter for 2 bolt would probably be the easiest.

I used a stock type kit from summit. The master cylinder on it now is for drum brakes. I thought dodge might have made something with manual disc and drum.

according to my wagner master catalog, the 73-76 dart manual disc brake master is part #F88787. it has a 1 1/32" bore.
the four bolt pattern should be the same to bolt up to your car, but i can't say for sure if that master uses studs or nuts and bolts for attachment.
you should be able to cross reference the wagner number to your master cylinder brand of choice.

Some front calipers require a lot of fluid volume so the 1 1/32" MC will work better than a 15/16".

All else being equal, a smaller bore master cylinder will yield a longer/lighter pedal travel with increased clamping force at the calipers.

For example, the pedal is lighter with a 15/16" master cylinder compared to a 1 1/32" master cylinder. The pedal is lighter with a 1 1/32" master cylinder compared to a 1 1/8" master cylinder.

All 3 bore sizes were used with OEM Mopar brake applications over the years.

If I understand your post, you converted to a dual drum reservoir m.c.. Now you added discs and the front is locking up. I think it is the residual valve it the m.c. It,s purpose is to keep about 10psi in the system. Good for drums, bad for disc. I just converted the power wagon to disc and removed the valve instead of buying a new m.c. I assume the summit m.c. is the same as a mopar. You need to remove the brake line from the m.c.. Find a screw that you can screw into to brass seat without damaging it. Pry the seat out. Remove the residual valve behind it. Reinsert the seat and brake line.

You now, I thought the OP meant the pedal was hard to push, but reading your reply about residual valves I can see that he may have meant the car was hard to push.

So, OP, which is it?

The master cylinder needs replaced anyway. Since I converted the front to disc the car is hard to push in the staging lanes.