Marlin's 450's are 1848's. I believe he has them on 1" spacers to create plenums under each. A little unusual with 2 individual plenums and two individual primary carbs but it obviously works.
It's a little trick to make the TR work like a street manifold. Don't look up his sleeves. You don't want to know what else is up there?
The 2x4 is a nice set up. In all honesty, I spend most of my time just trying to hold on to the steering wheel and not fall out of the car when I hit the accelerator.
The carbs are mounted backwards on mine. Seems to help with the sloshing issue?
#25's are the right shooters with no tubes. 31's should work but you should be happier with the smaller ones.
Ford put 31's in the 428's. It's a bigger mass to get going off of the line.
You need a constant 6-7 psi. An electric fuel pump pressurizing the mechanical pump is what I do.
With that you can set a consistent float level.
You should have the vent extension whistles and jet extensions. Those will help on hard acceleration.
I wouldn't challenge Marlin to a grudge match race. Those setups boogie. It's probably an honest 12 second car?
I am wondering if you have the cable installed properly on the carb? Did you test it to make sure that you are getting full opening at full pedal travel?