Exhaust change. How bad will it be?

I have a 68 Formula S 340, 4 speed with 3.91 rear. Engine mods are..... Ported X heads, balanced, 11.5 compression, Edelbrock Victor 340 intake, Holley 750 puddle dumper, Crower Street Roller (#31406 lobe center 112?, advertised duration i&e 290-294, duration @ .050” 251-256, lift .550-.555.) RPM range 3200 to 7200, redline 7800+.

I run headers and 3” exhaust with DynoMax Super turbo mufflers.

My question is...I want a more stock look under the hood so I am thinking about putting the stock exhaust manifolds (I have the original 340 HP parts) back on with the full (tail pipes) TTI, 2 1/2” Xpipe system. How bad will this be with my above combo.

Part 2. I know the exhaust change will hurt power, but what kind of a loss will I be looking at if I also put the stock intake and Carter AVS back on.