Chambered mufflers will mask the lope vs a straight through muffler.
I noticed when i went from my flowmasters to the Borlas, the lope was much more pronounced with the muffler change. Still, it should be somewhat noticable even with the flows. Are you sure it's a B303? Did you buy it new?
Was hoping you did not retard the timing on the install as my buddy tried that with his B cam year ago and did not have the supporting valve train. Basically he was thinking it would make more of a chop at idle and all it did was move the power curve.
Lope or chop is based on the amount of overlap the cam has. This is basically when the exhaust valve and intake valve on the same cylinder are both open at the same time. The more overlap the more lope or chop. I am not going to get into cam design as I barely know enough to talk shop with my engine builder who had a lot of input on the cams in both my cars (Ed Curtis in the Coupe and Comp Cams in the T-Bird). The Bird has some F'n chop but that motor is a level up from the one in the Coupe. These will give you an idea (buzz you hear are the twin Mallory Comp 140's):
Walking from the driver door to front of the car at idle:
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At the back of the car:
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Realize this car has a complete custom exhaust with 1-7/8" primaries into 3-1/2" collectors, 3-1/2" X-Pipe and a 3" MAC cat back for a Mustang modified to fit the Bird. Cam is a hydraulic roller with 0.598 lift with 114 degrees of lobe separation.
So with all of that I am going to say you have what you have unless you change the exhaust and/or add some compression.