pipemax is your friend.

http://www.maxracesoftware.com/

I have been building and racing mopars since 1974, I would never buy or build a set of headers without running it through this program first. Just me.

I would, and do use the larger pipes as you may want to hop the motor up more later like stroker short block, you can't hurt the performance of any BB Mopar by having to big of primary pipes on your headers, same thing on the exhaust system, 3.0 pipes and muffs are way better performing on the street and at the track than a 2.5 inch system is
Been there, done that
One of my old street and strip A body BB cars with a set of Hooker 1 7/8 inch fenderwell headers hooked up to a 2.5 inch head pipes and muffs and 2.5 inch tail pipes exiting in front of the rear tires ran 12.23 ET at 109.7 MPH with the exhaust system on in the 1/4 mile, I removed it and it ran 11.39 ET at 117.6 MPH
My last pump gas street and strip BB A body car with the same headers and a 3.0 pipes feeding into a set of old style Magnaflow 14x7 oval muffs with 3.0 inch in and out exiting at the rear bumper ran 7.01 ET at 95.3 MPH in the 1/8 mile, removing the entire system made it run 6.98 at 95.6 MPH in the 1/8 mile
Same thing on engine dyno testing, putting bigger headers on BB Mopars I've tested made the motors make more HP and Torque

my 1965 coronet they only make two header tube sizes 1 3/4 or 2inch its 440 with 240 trick flow heads it has 1 3/4 now would u switch to two inch

i use the big headers and pipes by "default", not choice. i use the tti recommended 3" dyno max turbo reverse flow mufflers. after getting parts together i had a conversation with a dyno max engineer, and at that time dyno max gave flow numbers on their mufflers, the engineer told me a 2.5" ultra flow would flow more air than the 3" reverse flow. my car is strictly a street car with a modified 440, sometimes no more than 20 miles a trip. the exhaust system is always wet internally from condensation which rots the mufflers sooner than i like. acidic water is a by product of combustion and can't be helped. now if i only did longer trips at higher speeds i could probably burn the water out of the system and everything would last longer. i do some road trips but not enough to keep the system dry. i seriously doubt that a smaller system with freer flowing mufflers would hurt performance much if any on what i have. plug changes would be a little easier with 1 3/4 headers, but with the angle plug heads i have the 2"'ers aren't bad. either way you go you'll like the tti stuff. you'll just have to decide what's overall best for what your doing, and don't fall into the trap of bigger is always better.

well just getting some advice car runs well 11.55 @ 116

my car with the big exhaust and other junk added to it wouldn't run 11.50's if the pope threw a gallon of holy water on it. maybe look at other things for horsepower.

my car with the big exhaust and other junk added to it wouldn't run 11.50's if the pope threw a gallon of holy water on it. maybe look at other things for horsepower.

well just getting some advice car runs well 11.55 @ 116

well just getting some advice car runs well 11.55 @ 116

If it's a street/strip car I would be fine with the 1 3/4". If it's an all out race car than 2" MAY help. Another thing to consider is, you said you went 11.55. Does the car currently have a roll bar? If it doesn't, your only allowed to go 11.50 without one. So if you change to 2" headers and pickup, now to be legal you'd have to install a roll bar which more than likely will slow you down what little you might of gained.

Is that with open headers? If not, what is the rest of the exhaust system?
And
What does the car weigh?