Hustler had a thread going regarding a lot of this but as I just messed around with it I figured I would make a thread that was very clearly titled for future searchers and put some info in here.
So far, the biggest suggestions from everyone on improving the steadiness of the MAP signal as seen by the megasquirt, (or whatever ecu one runs) have been to use a very small inner diameter vac line, change the source of the vac from the front port on the intake manifold as suggested to a T into the little line between the intake manifold and the fuel pressure regulator and/or to add an in-line fuel filter to the line going to the ecu (apparently one intended for a lawn mower is a good choice).
My vac line that runs to the MS is pretty small, without an actual number I'd say it is 'one step smaller' than the general run of the mill vac line - small enough that getting it to fit over the port on the IM takes some work. However, up to now my line went to the front port on the IM. Here is a datalog of said setup in a 3rd gear pull:
Using the same vac line, I ran it instead to a T fitting in the line going from the IM to the FPR. Here is the new log (3rd gear again):
Though not a jaw dropping difference, the MAP clearly doesn't fluctuate as much as it did, which seems to allow the computer to keep the AFR steadier, which is nice.
The fuel filter I found after searching around has ports for 1/4" ID line. I could not find any smaller ones so as of now I've not tried the fuel filter solution but am decently content with the improvement from this new vac source.