I don't have the freeware B-47 you are using, but from Alphasim B-47 .cfg as an example:
[Reference Speeds]
flaps_up_stall_speed= 155.700
full_flaps_stall_speed= 135.500
cruise_speed= 452.000
max_indicated_speed=456
max_mach=0.875
Exceeding either of these values (in your .cfg) will result in overspeed.
Your speed at any given moment in the sim can change with throttle movement, weather changes and especially wind shifts; that's the usual culprit.