If you do have a completely stock '56 Loadomatic dist and carb that were properly rebuilt and matched then you should check the shop manual for the proper vacuum curve.  It will peak at something like 3" at which you should then have the full factory advance. I don't have this vacuum/Advance curve handy but It is in the '56 shop manual.

As Gary and Doug both mention, the first item to check is the distributor design.  If you have the Load-O-Matic, then it needs to be replaced with the ’57 and up design.  The LOM’s have no centrifugal advance mechanism in them and rely solely on vacuum from the carburetor to make them work.  1956 was the last year for the Load-O-Matics and starting in 1957, they were replaced with the more conventional design that incorporates both centrifugal and vacuum for ignition advancing capabilities.  The modern carburetors such as your particular Holley do not have the internal circuitry to allow the Load-O-Matic distributor to function as originally designed and typically throw too much advance at the engine under cruise situations and no advance at initial take off and under moderate or heavy throttle.  If you have the distributor vacuum hooked up to the intake or to direct vacuum at the carb, then the vacuum signal during cruise situations is intensified which compounds the surging problem you mentioned.

The LOM distributors measure about 3½” at their housing while the ’57 and up distributors will measure out at about 3.7”.  The vacuum advance chamber on the LOM distributor will be about 3” O.D. while the ’57 and up distributors will be in the neighborhood of 2½” O.D.  The LOM distributor will have a pair of small springs located above the breaker plate while the ’57 and up distributors will have the advance springs hidden and out of site under the breaker plate.  The LOM distributors used straight breaker points while the ’57 and up distributors use curved points but with you having a Pertronics conversion, this will not be much help in your particular case.  These tidbits may help in identifying which distributor you have. 

In your case, the distributor vacuum needs to be hooked up to ported vacuum for normal running as being hooked up to direct vacuum would be part of the surge problem you’re experiencing.  I’m assuming you’ve timed the engine with a light?  If not, then that would be next on the list and also check the advance curve with the light while bringing the rpms up.  Initial timing can be as much as 10-12° with the vacuum advance disconnected while the total timing with the MSD distributor is not expected to exceed 38°.  Total timing may all be in by 3000 rpms depending upon how your particular distributor is set up.  This is something you can verify with the timing light by simply bringing the engine rpms up until the timing advance stops and noting the rpm at which it stops moving.  The MSD distributors have a considerable amount of flexibility for adjustments to the advance curve both in the amounts and the rate and this is something you may have to dive into deeper if the surge at cruise is still an issue after addressing the aforementioned items.