We, as every other FED with SBC that I see use the front water pump bolt holes for front motor mounts, that are clamped unto the chassis. And the rear motor plate bolted to the chassis, but some guys also clamp the rear motor plate to the chassis. It is a good idea to have the front motor mounts line-up with a chassis upright.
1'st photo is our front mounts, 2'nd photo is the rear motorplate, including our rear support mounted to the motor plate that helped cut down our tire shake, that with the rear trans mount keeps the chassis from flexing between the motor and rear end.
I'm surprized you would have a problem with it moving forward and rearward if the rear motorplate is bolted to the chassis.
Jon