Seriously!!!

You can internally balance your 360 crank assy but it will always be that way and any parts replaced afterwards will have to be dealt with.Since you already have the converter and want to use it you will need the 318 balancer also.Tell them you want them all balanced together and they should do it but I think they are leary about doing it because its not the normal way to build an engine as most stick with the factory spec parts,mainly for "down the road repairs".