340 LA motor mounts

I have a 70 dart which is being converted to a 340 4 speed car. Been working a long time and now assembling, ran into a problem I didnt expect. I installed a newer k frame with the new style, drop in, motor mount style. The motor mounts that came with the kframe dont fit my 1970 340 block. The mounts have holes 3 1/2 inches on center and the block is 3" on centers. Anyone know where I might find a mount which will mate my motor to what apears to be a 1974 kframe? Thanks in advance for your help on this.