The TQ is rated at 800 or 850 CFM, if your TQ has a 1-1/2 inch primary, you have the larger. The small primary side allows for good velocity passed the throttle blades.
The secondary side has throttle bores of 2-1/4 inch.
Being that the secondary side has a spring loaded air door adjustable via a pair of screws, this allows the TQ's secondaries to be adjustable to open when it is most advantages for you. (AKA, If you bog or run slower in the 1/4, tighten up the door a 1/4 turn until performance improves.)
A TQ is the most flexable carb. It is not the most adjustable, tuneable carb. It's flexabilty is why it is so love because you can mount it on dang near any engine and get very good performance. High compression, low compression, little cams, big cams & even race cams.
Once you get a good TQ & it dialed in, your set.
On a squarebore carb, the barrels are generally speaking, equal in size. There maybe a slight offset between the front and rear.
Square bore carbs that get larger in size tend to be a little big on the primary side for low compression engines. Adding a big cam can further the problem by creating less vacuum, less air flow, slower velocity passed the primary side of the carb.
Notice how on your Edelbrock 800 the primary throttle bores are simple bores with zero Venturi to them. That is the biggest issue you have with that carb on your engine. Go up to the last sentence in the above paragraph. That so applies to your engine carb set up.
This 800 is best suited to a big engine. And/or one that has all the heavy street strip parts that can take advantage of a large carb. A nicely put together (street- street strip) 440 or 500 could use this size AFB effectively.
A small engine, like your 400 is having a problem generating enough air flow past these huge throttle bores. In order to correct this, you will have to start changing a few things which leed to other issue of a to large carb. It becomes a pain chasing your tail around in circles when the carb is to big.
Through years of trial and error, a thing figured out why before I was born, carb sizes and perameters on most any given engine and performance level was generally figured out. Having the misfortune of carb issues on various carbs through out my time on cars, I can tell you these guys are right.
While I have had a 850 Holley on a mild 318, in a pinch, and tried to use it for a while, all I can say is it really sucked getting that thing somewhere live able until the 600 came in.
Everything about your engine build and the drive train behind it screams "I'm a driver!" All parts should be balanced. There is nothing about the description that would lead me to a carb over 650. While you could tune in a 750, the smaller carb will have crisper throttle response and allow for room for more mileage.
I hope I helped a little bit. Just remember the engines performance is combo dependent. The better the combo (balance of parts) the better it will perform.
