For the engine I just used the standard MGB rubber mounts and these seemed to fit fine. I assume, like you that the spacers would only be needed if there was extra room and you needed to avoid the mounts being 'stretched'.
As far as I know there is only one engine plate on the front of the block.
For the gearbox, again, I just used the standard MGB rubber mounts - none of the other brackets - and fitted it to the NG cross member which bolts to the chassis.
For further clarification you might like to look at my web site under Build Stages - Engine and Transmission. See www.ngkitcar.co.uk. If you would like any other pictures please email me.
Best of luck.
Chris