I was able to move forward quite a bit over the weekend. In this session, I trimmed the longerons at the firewall. Van's has you make them about 3/16 inch too long and them trim them back in this step. It took a couple of tries and then some filing, but I think I got them perfect. Below is a before and after pic. After the second pic, I had to file it back a bit more to allow the firewall to be set back far enough.
Next on the firewall, per instructions I drew a center line on the bottom aluminum flange to use as a guide to set the distance of the firewall from F-704. Then slid it in and measured 5/8 inch back from the blue skin all the way around. Looks pretty good, so I drilled the two outside holes and put in clecos to hold it. Next the instructions say to "take a brake" so I am going to put it away for tonight. Next I come back and drill the firewall to the skin, which is a fairly important step.. So will come back, re-measure everything, then go for it..