+1 for the firewall insulation before hanging.
+1 for the wheel pants and gear leg fairings. The only thing you won't be able to finish are the top intersection fairings until you have the cowl in place. I'd leave the cowl work until you have the engine hung and prop in place (specifically the backing plate to determine the cowl alignment).
I'd wrap up everything you possibly could on the airframe (canopy, paint, interior etc) and firewall before you hang the engine, including but not limited to:
Firewall pass throughs for engine sensors (one on the left and right)
Firewall passthrough for the main bus wire (should be kept sensors)
Alternator shunt and ANL fuse block.
Main bus fuse (if you choose to install one, which I did because I have backup batteries in my avionics).
Engine controls
Start/Master relays (bolt pattern for the BlueSeas 9012 low draw relay is different from the Van's relay, should you choose to use one)
Engine sensor manifold block - drill the holes but leave this off as you will need to add the fittings.
Heater box.
I wouldn't (and didn't) do any avionics at this stage, given that they go out of date so quickly (apart from installing cutting the panel/sub panel for radio racks, installing shunts and autopilot servo). That said, it's good to have an general avionics plan at this stage so you can decide on mounting configurations. I even purchased some racks from Stein early to achieve this fitment task, but just held off purchasing the expensive bits.
More importantly, I would be thinking how you will be preserving the engine after you have removed it from it's shipping preservation (if it comes from Lycoming, it will be in a sealed plastic bag with desiccant in it, keeping it in a perfectly low humidity environment). Once it's out, the clock starts ticking. I worked pretty much full time on my aircraft and it still took me 10 months from the time I had the engine hung to the time I had the aircraft completed and cranking over. They say in the lycoming instructions that you shouldn't move the crankshaft as it will wipe the preserving oils off the cam surfaces. Fine in theory, but when you are trying to fit the prop etc., sometimes you have to rotate it. The easiest way to mitigate any subsequent corrosion risk (assuming you don't live in the middle of the desert somewhere) is to install desiccant spark plugs (one in the top of each cylinder), seal off the intake and exhaust, and make a dehydrator for circulating dry air through the crankcase. The one I made is just a 2W fish tank pump that I modified to suck air from a hose instead of from beneath itself, which in turn was drawn from the desiccant box through a modified respirator filter (to ensure no desiccant dust gets in the engine). Use the orange desiccant, as the blue stuff is a know carcinogen. The air gets pumped into the dipstick port and drawn out the crankcase breather and pushed back into the desiccant box through a fish tank air stone, but first via a jar with a humidity sensor. It generally keeps the engine at 10% humidity or less. At that level, even with no oil on the cam, you will not get corrosion. Now that I have the engine running, I'm still using when I know the engine will sit for more than a few days. It's interesting to note that about 45 minutes after shutdown, then engine can still be at over 110 deg F, and the humidity 75%. When the engine cools down, the air will normally reach its saturation point, and the moisture will be forced out of suspension and onto your metal components. Even with my miserable 2W pump, I'm able to drop the humidity at a rate of 2% per hour, until it settles on this sub 10% level. The engine has been run about half a dozen times now, and with this and 10 months of non stop work, the desiccant still hasn't come close to reaching saturation. When it does I will just need to put it in a metal tray in the oven and recharge it.
I hope this helps.
Tom.
RV-7
Awaiting final inspection.