Use an actual vacuum gauge to verify , could be a sensor bad but I doubt it . I was looking at one of your earlier logs that you sent me and it didn't show a negative MAP value while I was looking at it . Might be another glitch in the 7747 data stream and the XDF file you are using to view it. Which .XDF are you using ?

MAP values generally start high (100) at high vacuum and decrease as engine vacuum goes down (0) Usually on a N/A engine you will not see true 0 MAP .

MAP values on high performance or altered engines can be useful to track down intake tract restrictions , such as with TBI engines .

One note : As for the engine , you have a Gen1 small block , not a Gen II , Gen II small block Chevrolet is the 92-96 LT1 and LT4 platform , Reverse flow cooling , gear driven water pump , Optijunk distributor , etc.... Might save some trouble down the road asking for parts as aside from them also being 350 CID there are vast differences.

TOM