Doesn't matter if pump is before or after boiler because the pump is a circulator just moves water around rather than like a well pump that takes a few pounds of pressure and raises it 50 to 100's of lbs. There is no problem if the pressure is a few lbs high. Boiler relief valve with blow off at 30lbs.
Heres a chart for working and bursting pressure for black pipe.
Theoretical bursting and working pressure of wrought steel standard, extra strong and double extra strong pipes.