The B2 series of filler metals are intended for use with PWHT, and high temperature applications. Since PWHT is not normally done on chassis, then B2 will be hard and brittle.
With HSLA steel containing Cr and Mo, you are not trying to match the chemistry. What chemistry works well for base metal often doesn't work very well for filler metal.
The 309 does have significantly more Cr than the base metal, but it also has significant amounts of Ni, which render it austenitic, and therefore ductile and crack resistant. The 309 seems like it could be overkill, but compared to the price of a chassis, overkill on filler metal will cost you pennies compared to the cost of a failure.