You can make a temporary baffle to block off about 1/3 the exhaust pipe outlet to see if there's any improvement,a piece of sheetmetal and a hose clamp will do it.
When you change one thing,it may screw up the engine tuning.Been my experience that the old 6's,once modified,especially with a hotter cam,need more spark advance at mid speeds,like a recurved distributer advance.For a test,simply advance the timing about 10 degrees,road test it.Temporarily over advancing the timing won't hurt anything,just don't run it at full throttle and maxiumum revs.
