If it were me and you have to run the smaller headers you need to up the compression and reduce the overlap, build more torque sooner and lower the RPM peak...Kinda similar to if you have FAST Racing type exhaust restrictions....

As small as 1 7/8" x 3 1/2 with a 3" system sounds, I'm sure some of the FAST guys limited to Manifolds and 2 1/2-ish(lol) would kill for the ET gains they might make having the exhaust system you're working with.

Also the 40-50HP difference/advantage with open headers on a Dyno is likely to be considerably reduced in an actual car with full exhaust pushing against rolling and wind resistance.

To me a big C body is going to ET better on FAT mid range torque, it's going to make it's best moves through the middle and on the shift recovery and it's simply not going to top end charge like a lighter car with a lot less frontal area.

You want a big grunt motor not so much a 'horspower' motor.

I got a ported MW Indy Dual plane if you want to borrow it to try it out. I'd run it over a single plane on something like your 68

Unless it's in a really light car like an A body, to me a BB Mopar is less like a Hemi or a Small block Mopar and more like a Buick/Olds/Pontiac type motor, and the ET's respond to a similar kind of race tuning.

Also if you look at your exhaust sytem I would put the 3 1/2" collector on it and bring it straight back and run your "street" system coming out of the collector extention on a 45 degree cut out branch on each side. Forcing your exhaust through that 'double dog leg ' just to get to the cut out is killing some torque and power.