I need to add some down thrust to my H9 Alpha 40. Before my crash, I had it trimmed out pretty well for level flight between 1/2 and 2/3 throttle. Now, after the crash, I trim in all the down elevator I can, and it still wants to climb at 1/2 throttle. I've adjusted the clevis on the elevator up to the point where I don't think I should give it any more (I don't want it to nose down at idle speed during landings).

What I think happened was that when I reconstructed the firewall, I didn't put enough down-thrust into it, so now I need to compensate another way. Can I just put a washer or 2 behind the upper 2 engine mounts where it bolts to the firewall and call it good? Or is there a better way?

That is the way I would suggest doing it. Another option IF THE ENGINE IS MOUNTED VERTICAL is to put washers under the rear bolts of the engine between the mounting lugs and the motor mount.

FWIW - In the fastener section of your local hardware store you will find a variety of different thicknesses of washers.

Campy- the Alpha uses the std RTF "clamp" style of engine mount. He would probably be best to put the washers between the mount and the firewall. Add thickness gradually so as not to overdo it. Would probably also be good to use larger diameter of washer so it does noes not just get squashed into the firewall.