Vacuum Tuning

You didnt mention what your initial timing is.Try getting it up to 16/18/20 degree range.
Cams of this size(240*@.050), like idle timing and/or idle air. 10/11 inches vacuum at 900rpm with 18* seems about right. The 30* on decel you mentioned seems pretty high, leading me to believe that youre trying to get it to idle very slow, or the guage is off.IDK Once you get the timing up, the stink seems to go away. Or nearly so.More timing = more vacuum,to a point of course.
In bouncing up/down, that gauge is telling you whats going on in the manifold. Its not lying. That cam is allowing pressure pulses back up.It is after all on a 108*CL, and with 64* overlap. If you installed it with 0* advance, the intake wont be closing until about68*ABDC. Thats pretty far up, so compression could easily,at idle speed, be upsetting your gauge. Your gauge needs a damper/restrictor on it is all.And maybe,a bit more timing/idle speed.
Enjoy the new set-up.