What's it going in?
Better be a small one if you plan to get it to idle in gear.  Those Thumpers are SNOTTY.
I would 'guess' an 8" converter.
Really need to consult a tranny/converter vendor. May do better to get one custom made for your application.
(Don't forget an external cooler too!)
6sally6
PS...........I ain't seen you on here before...............you new?! 
If so welcome!

Know of any good vendors that make a custom one?  
It's going into a 1966 Mustang coupe that I've owned for last  25 years.  (Drove if for 5, and spent last 20 years in pieces, now getting put back together).   I haven't looked at the cooling system yet either.  What do you suggest, a separate cooler for the tranny, or an integrated one built into a Rad?

I've been collecting parts over a number of years, while the body was getting fixed at the shop.  (New floors, torque boxes, subframe connectors, rockers)...   Some Custom work was done on the rear, (Shelby style trunk (small stylish spoiler built into the trunk.),  recessed shelby style taillights, etc.....   and a spanky paint job to go with it.  (I'll need to find a hosting site for pictures so I can add a few peeks)

The engine:
I had polled the engine out of a 93 thunderbird, and had it sitting in the garage for a few years, until I had the chance to get to it.  I was going to just get the engine re-ring, and new seals, but when the builder went in, found water in two of the cylinders, so we decided to go and tear it down for a rebuild.  New Crank, new pistons, new cam, new intake, heads.
Brakes:  Mustang Steve GT2008, modified pedal for power brakes, and booster/MC from Mustang steve.  (Bought back in 2008).
Suspension: 4leaf mid eyes springs, 1' lower coil springs, roller spring perches, and roller idler arm, adjustable strut rods, camber kit from (all from open tracker racing).    
Small bearing 9" rear end.
Tranny: (4R75W from a 2004 mustang, with stage 2 shift kit installed). or AOD.  (I have both cores, not sure which one to use.

I used to be on the site back in 04,  collecting information, asking a few questions here and there.  (Pre this fancy site upgrade).  And  just recently re-joined in the last few months asking a few questions opinions.
The car has allot of sentimental value, it was my Dad's at one time.  My father in-law started the custom/resto work, but passed away before the project could be finished.  Now my mother in-law, brother in-law, nephew, and their shop staff are completing the work.

 

I went to a place in Buffalo NY that custom makes converters, Competition Transmission and Converters.
They had a form I had to fill out with the cars details, weight, gear ratio, tire diameter,  vehicle usage and some other info, IIRC.
He needed all that info so he could custom build it for me. The converter was internally brazed and balanced to the “nth” degree.
I think a place that needs that kind of info is better than getting one off the shelf so to speak.