2016 148MR Cargo
I know the post is older but want to point out that L-track doesn't have one weak point, it has four. They are the structure beneath the cabin floor, bolt strength, bolts tearing through the floor, and deformation or L-track twisting at the connection of the seat to the track.DOT standards for seatbelts work out to a 5,000 lb pull strength. This can be achieved after-market with a 7/16-20 bolt in an installation where the bolt is the failure point. On the other hand, on older Honda CRV's the back seats were bolted down with four M8 bolts.
Your L-track should have a pullout strength spec. I would guess that the L-track is the weak point of your design.
L-track started out as "Logistic track" for the airline industry to hold down cargo and allow seat spacing to be reconfigured in different cabin lay-outs. However in the early days there was a problem referred to as "Sikorsky ejection seats" where the seats and track would pull out of the floor. The term came from Sikorsky crashes in Viet Nam. In the 1980s we were working a major aircraft accident more than once a month and it was not uncommon to arrive and find some seats with bodies piled at the front of the cabin or at least folded over forward crushing the person into the seat ahead. Where the cabin floor would twist or buckle near a rear seat leg was a pretty common place for the foot to come out of the track and the seat to fold forward because the rear attach is mainly tension upward as the seat tries to pivot on the forward mounts. The NTSB did a major study of the problem in the mid 1980s and the FAA accepted a recommendation to require the seats remain attached through at least 16 Gs and they added language about floor deformation too.
As I remember from the old timers the 16G figure picked up by the NTSB for the FAA came from DOT impact sled tests in the 1960s and it was based in the standard weight for a person back then being 170 lbs. Multiply 170 lbs times 16G then apply a 2X safety factor and you get 5,440 lbs, which was rounded down to the 5,000 lb figure.
Bottom line for van builds is to make sure there is enough beef beneath the floor for the L-track to stay bolted to. Simply bolting to a nut on the bottom of the sheetmetal can result in the bolt tearing up through the sheet even though the bolt doesn't fail. Note that the fail load for a steel Grade 1 (Class 4.8) 7/16x20 is only 3,917 lbs while a Grade 8 is 10,674 lbs so Grade matters a lot. Most cars/trucks/vans don't have the twisting and floor deformation we'd see in airplanes and there's not much we can do as modifiers to improve it.
To carry motorcycles I put L-track in the beds of my pickup trucks, I add L-channel beneath the bed, use black oxide csnk bolts (130ksi vs 70ksi for stainless), fender washers between the nut and L-channel on the bottom, then paint over the fasteners for rust prevention.