Dave
Just be glad you don't have to press "2" for English.
http://www.youtube.com/watch?v=zQ_ALEdDUB8
http://www.youtube.com/watch?v=6hqFS1GZL4s
http://s73.photobucket.com/user/steemtrayn/media/MovingcoalontheDCM.mp4.html?sort=3&o=27
Your load will look best if you hunt some pics of loaded gons and copy the way the trusses are secured in the car.
IIRC the largest Plastruct truss is about 1" = 25.4mm = 7'3" in H0... which would be a good size load.
Trusses have their main strength up and down through the web... so they tend to be carried (and loaded/unloaded) in their strong position - on edge... which is great for us as it looks better. If carried flat/on its side arrangements would have to be made to support the truss so that it didn't get a curve set in it... that said... they can be carried both flat and - to keep them in height limits - secured at an angle on supporting frames.
Loaded in a normal standing position trusses would sit on timbers... probably wedged square across the car and possibly with lumps of timber nailed either side of the bottom flange of the girder to stop it shifting sideways. It's possible that there may be some lengths nailed to these and through the gaps in the truss... BUT such a large lump of load would be held down by chains (or nylon ratchet straps in a modern loading). The ends would also be blocked to stop the load shifting and smashing through the end if the car stopped suddenly. For this reason you might find some of the chains were run at an angle from about mid car to the end tops.
The end would not be broken out of a Gon as this would cost a repair and weaken the car. Gons did come with drop ends to allow over length loads. there is a limit to how much over length you can go without fouling adjacent tracks. On models this tends to be a greater restriction than the real thing because of the sharp curves we use.
If you can get trusses with rivet/bolt and/or any other detail the load will look better.
Presumiably your trusses will be new so they will want nice new paint rather than weathering... but you could add on delivery marks and codes for where they bolt/weld together on site when they arrive. This isn't so difficult to do as it might sound... the lettering will probably be very small... so just cut a line or two out of the letters from a decal sheet left from a boxcar or whatever and apply that... it doesn't even matter which way up you put it - most times it will be far too small for anyone to read. The fact that something is there will look good though.
OR... as I posted in the "other loads for gondolas" thread you could have scrap trusses that have been cut to fit the Gondoals after being ripped out of wherever they were...
I don't have a clue how to calculate the weight of a truss (can anyone guide us please?) BUT a lot of it is air so it may be possible to load two or even three trusses side by side in one Gon... what you MUST be able to do is secure them and get them out...
Two welded together with short(ish) cross plates would at least make a change from the quite common single truss that looks like it has come straight from the LHS and been dumped in the car without any further thought.
You mention other Gons... so developing some idea of a combination of loads could be good...
You might have two cars with a truss (or pair of trusses) in each with squared ends facing each other - with plates/holes for bolting them together - and angled ends at the outer ends of the loads.
OR You might have two (or four) half car length trusses as above making up one car load. (This could happen where there would be issues getting a full length truss into place on site)
Again... you could have the last with another car carrying the square-at-both-ends middle part of the span.
Trusses are not always supported only at the ends. So you don't have to limit the total truss length to what could be self supporting... which means that you can build up these combinations of load.
If you can figure out a reasonable logic you can start to add extra bits to the load.
For example the final structure may have various braces or cross girders attached to the truss(es). If the truss doesn't make a full load by weight it makes sense for the extra bits to share the car for the ride... so long as they can be secured and loaded/unloaded safely.
You could even add a crate/box of all the bolts and bits of metal needed to join things up.
Hope that this helps.