Reviving this old topic for a new breakthrough!!
They patched the old bug I used to go out-of-bounds, so we are going to have to use weight to determine the speed relative to other builds.
The OG build was 2940 kg, which was light, but not light enough.
The second version completely changed, I got rid of the carjack because at that speed it was useless, and then I changed up the gun position.
You might notice (if you look well enough) the armor to keep the barrels in place is still made with heavy parts (not like HEAVY heavy, still light, but not optimal), and the lighter the better. I am proud to announce the newest version, which is the lightest so far, and fixes the armor issue!!
The guns are mounted on the ammo, which is bad for normal builds, but nothing about this is normal. The ammo is 4 wide, and the cabin 6 wide, allowing the barrels to use the cabin as part of the armor to keep them in place!! the rest of the “armor” is shoot-through, and the lightest variant of armor I could get. They are connected partially by headlights, which are even lighter (by 1 kg, but it’s better)!!
Frontal view to see the guns on the ammo:

Since this is the lightest build yet, theoretically it’d also be the fastest, right?? Well… no. 
The physics in this game are wonky after 2000 km/h, and everything wobbles uncontrollably, making it hard to pass that limit. I only got to 2040 km/h before the wobbles brought me back down, so it might not be possible to get very much further, even with a lighter build. This might have been patched, but it is unlikely, however we will never know since they also patched the out-of-bounds exploit I used.
This was a fun experiment, I still might use this build on a bigger bedlam map so I can fly in circles at high speeds. I’m not sure what else to use it for. I would fuse every part for mass, but that’d be a waste of coin I don’t have, so I think this is it.