When I installed the engine mount the holes in the firewall were close but not perfectly aligned with the engine mount. I suggest waiting until you get your engine mount before drilling those holes.
There use to be several suggestions for temporarily attaching an automotive engine stand to the fuselage. A search here may find some of those, although many of the photos have gone offline. One method used two pieces of wood bolted together that sandwiched the firewall and support angles. The bolts went through the wood in the opening in the firewall. An engine was then bolted to the wood. You need to support the tail, but that can be done with an old office chair, which can be raised or lowered as needed.
Here is a review of a commercially available option. This one uses the per-existing holes and bolts to angle.
http://www.vansairforce.com/communit...ght=rotisserie