The T bird steady rests are absolutely necessary in the T Bird chassis. When I and my car were much younger I broke them both by sliding off an icy road and hitting a curb. As someone described above the engine just flopped over with the slightest throttle opening without the steady rests. So you definitely need them.There is another current thread that has some discussion of the span of the engine-trany mounts. It was appearantly a FoMoCo design criteria for the Y that the engine-trany assy be very stiff because the 49 and up flatty with its front mounts and the trany with the undermount had a pronounced sag! Ford engineers used oportunity of the Y's deep block skirt to locate the two lower bellhousing bolts below the crank centerline giving the whole engine bell housing trany increased stiffness.
It would also be instructive to take a look at the truck bell housings which have a pair of gussets running to the two upper bolts. They are likely there to support the tension load of the tranys hanging off the back of the engine.
It would be my opinion that the block itself can stand the span issue. But the real question is if the flatomatic bell housing has been designed (or tested in a lot of applications) to live comfortably in an application with a long span between the front and rear mounts.