Hi Max,
The only trick to the horiz. alignment is knowing where the play is. With mounts, you want to know whether there's clearance between the engine bolts and the mounts or between the mounts and the studs in the bed.
In my case there are no resilient mounts - just oversize holes in the engine. with the upper nuts and washers removed, I just pry the engine against the beds. Once aligned, I pour a little Bondo between the stud and engine, then install the upper washers and nuts.