I think you should weld caps on the ends of the few tubes which are open like FF1, FF4, and there are a few others on the chassis (total of 12 iirw). Otherwise moist can enter the tubes which will be likely to get rusty from the inside.
There is a quite easy way to do it using small pieces of 19/20mm square section tubes you used for the transmission tunnel.
, sur Flickr
Even though I haven't use this technique myself on my chassis, I have tried it and it is soooo easy (I got it from a member on this forum, but after
I did mine with small square plates I had to cut one by one... I actually regretted I hadn't discovered this good idea myself or known it before
I've made a thread on my blog about that, perhaps it can give you some inspiration...
Very nice looking chassis BTW, very tidy