Firewall / Rotisserie question

Planning on using an automotive style engine stand as part of a rotisserie for building the fuselage (RV7, but should be a relatively generic question). People seem to be using the engine mount holes for mounting the firewall to the stand, however the fuselage instructions only call for the top two holes to be drilled at first, (I’m assuming match- drilling the bottom two holes w/ the actual engine mount.). Is it ok to drill the bottom 2 holes in the firewall assembly using the two existing holes in the stainless steel firewall, or is there a better way to mount the firewall assembly in a rotisserie type setup?

Thanks!

The RV-6 construction sequence requires all 4 corners on the firewall to be drilled to enable the firewall to be attached to the jig. I drilled these holes undersize (1/4") to allow for final size drilling at the time of attaching the engine mount.

Careful here using automotive engine rebuilding stands for a rotisserie. They are usually built with a backwards tilt. Instead of a 90* angle from level, they are manufactured about 95* to compensate for the engine weight. Even if you shim to mount flat to the firewall, when you rotate it’s going to stress the heck out of the fuselage from the weight as gravity takes its toll. I solved the issue by cutting the upright support to a perfect 90* and re-welding.

Engine mount holes

Engine mounts are not identical. It's a good idea to drill the holes using the mount to insure they line up then use the rotisserie. I drilled mine small and got lucky later. The engine mount holes perfectly aligned.