For a 170cc head your heads are flowing pretty good. (251@.500" 255@.550") Yes they will flow more with porting. This makes the port larger. Larger ports want more rpm. Most all of the porting flow improvement will be at high valve lift.
If you want more "top end" you need to think along a total systems approach.
Your can is small for "top end power"
with 4.56:1 gears why are you limiting rpm to "under 6000rpm?"
more airflow will want more rpm. (same cid)
If you really want to see the gain from porting your heads: I would:
change the camshaft
.550"+ lift 244ish@.050" (hyd) 108 LSA
248 to 255 (solid lifter)
something like this
Victor JR manifold 750+cfm carb. (modify your carb or get a larger carb)
9" ATI converter 3500-4000rpm stall
And port the heads.
I would also improve the airflow of the exhaust system.
This engine will want around 6400rpm and will still drive well on the street.
You'll have a whole new opinion of your 351W.
You need to pull your heads and actually check your real compression ratio by measureing the variables accuratly yourself. 9.5 minimum required.
With the heads off its not that hard to do.
Even as they sit now your RPM heads are not the big choke point.
have you had this car to the track? ET?? MPH???