Hi John,
I'm installing an alt engine, but the mount at the firewall didn't change. I left all the firewall rivets out; just clecos down the sides & across the bottom. Installed the mount, hung the engine, did all the plumbing/wiring, etc. Then pulled the engine & mount to rivet the firewall edges. You *might* be able to do the bottom after the mount is installed, if you have a no-hole yoke for your squeezer, but it would be tough. Not sure you could get to all of them, even with the special yoke. IIRC, some of the ones on the sides will require the mount to be out of the way.
I didn't see any evidence of stuff moving with the engine hanging on the mount & only clecos holding the skin. Remember, the firewall itself is non-structural. All the loads (at least in the 1G static load situation we're talking about) should flow through the aluminum angles to the top & bottom longerons. I fit my cowl with clecos in the firewall perimeter, & saw no difference in fit after riveting.
Having said all that, you might not need to wait on riveting. Biggest variable for you would be how much shim you need to get a perfect transition from cowl to fuselage skin. I waited because of the alt engine; I was delaying everything I could, as long as I could, just in case I needed to modify something.
FWIW,
Charlie