Robert, The smaller pipes hurt a little, but unless your Mopar was specifically singled out for this restriction, everyone will have the same problem.

I can tell you that as the cubic inches increase (it won't matter if that 408 CI was because you stroked a 360, bored a 400, or destroked a 440, its still 408 CI that has to pass through the exhaust), the effect of exhaust restrictions becomes more apparent.
Higher RPM will also be more effected by exhaust restrictions. These restrictions will likely show up at the end of the straight on a circle track, and as the laps add up (unless you run a track where the off throttle time adds up to enough time to clear the exhaust buildup). An exhaust restriction also tends to push the engine temp on extreme conditions.
If you think in terms of how much exhaust has to pass through those smaller pipes, you will have a clearer picture of how that restriction will effect your goals. If you want big CI, lower your RPM range. Gene

A friend runs local short dirt oval,408" good comp,W2 heads and magunum manifolds! He does it just because he wins and the other racers(chevy guys) can't figure out how a lowly overweight mopar(300lbs or more heavier) can run that fast,he has to sandbag to keep most on the same lap. I'm building him another 408 but Im trying to get him to put headers on this one.Lol

https://boneheadperformance.com/extrude-honing/extrude-hone-pricing/

Years ago did not someone on moparts spend the $700-800 to Extrude Hone an iron exhaust manifold of some type?

I have pumped millions of tons of rock and clay through pipes which “hones out” the pipe at great expense. The wear is so great we would buy basalt rock lined straight pipe lengths, and buy from Georgia Iron Works “white iron” elbows.

Vast amount of sand, phosphate laden sand, and TiO2 laden clay is pumped in Florida and Georgia.

It always seemed to me that instead of the expensive Extrude Hone way, someone could just get permission to divert some of the mining “slurry” through parts that needed their insides smoothed out.

Sewage sludge is pumped most every city facility from large settling tanks to holding tanks.
It might also work but slower than mining slurry.

Though it is turbocharged, Dizuster has made 850hp through premagnum 1 7/8" truck manifolds.
Doug