Power loss. 318 Vs 340 manifolds

Not sure what you are referring to here but I run 340 manifolds with 2.5" and have plenty of room.
does your tube point forward before it makes it's way back. Ive seen 340 manifols that point straight down close the the fire wall, mine points forward toward the front left then the pipe has to be bent to go backward. I think I found a solution from TTI, The Head tubes for $178 + shipping.