Harmonic balancer tightening (crank bolt) engine in car with 4spd manual transmission

I did the balancer bolt on my asshole brother's 65 Buick Gran Sport 401 nailhead with an automatic by clamping a set of vise grips on the flexplate, rotated against the block, and torqued to 250 ft lbs. Piece of cake. My own junk I just pound it with an impact gun and call it good.