Kukla ... I don't remember there being any steps in mounting the engine that were difficult ... it is very straight forward actually.
For drilling the 3/8" holes into the firewall, I did use a DeWalt pilot point drill bit. The pilot tip fit the lead hole perfectly .... so it made a nice straight hole. You will want to make sure the steel mount stays tight to the firewall during the drilling and that the lead holes are aligned ... believe I used a pin to hold alignment while the first hole was drilled then inserted the bolt and moved onto the second hole.
Prior to installing the engine, do make sure your engine mounting bolts slide through the washers and spacers and mounts. I discovered mine did not at the time of trying to install the engine .... there was powder coating inside the hole in the washers that prevented the mounting bolts from being inserted into the washers.
On my build, the new muffler design required hogging out the hole in the lower cowl way beyond the scribe lines. Prior to making any cuts for the exhaust pipe, would suggest making sure the muffler assembly is centered under the engine (left to right) then have a friend help you lift the cowl straight up and mark where the exhaust pipe actually touches the lower cowl and work on creating your hole from that point. You won't need the big long slot scribed into the lower cowl because the exhaust pipe now exits the cowl vertically as opposed to the slant it had with the older muffler design.
My preference was to hang the engine prior to cutting and fitting the cowlings and that worked out quite well and having the engine in place did not make the work more difficult. Suppose you could leave the muffler assembly off, fit the cowls then attach the exhaust system and then mark the exhaust pipe hole in the lower cowl ... wish I had done that as opposed to cutting the long slot per the plans only to find out I had the new muffler design which really does not need the long slot.
Those were the main issues as I remember them ... all and all it was a painless process.