it's always from the point of view...
if you be in vehicle which goes 35 mph, your weight is about 150 pounds, and you decelerate in 0,1 second to Zero mph your body mass will became 1,5 ton....now here the seat belt starts to work...in a passenger car the seat belt is so designed that it allowed you to move forward...this increase the time you are moving....means it needs longer that you come to the stop than the vehicle.....and in this way it is reducing your mass...and this will be protecting you to get hurt.
in a LSR vehicle, with the seat belt we using, you became a part of the vehicle....only in the direct direction of the seat belt there is a stretch effect which reduce the stress on the body...sideways there is no movement possible....
now it's depends how you "crash"....pencil roll, this will be never an issue to your body because the deceleration force on your body is very low. if the vehicle start to fly it is totally different...every time the vehicle has a contact with the ground...as a part of the racer...the deceleration moment goes on your body......
if the impact to the ground is a instant stop (albeit for a very short time) your body will create a big mass as I explained with the sudden stop inside of 0,1 seconds....and this is the critical moment.....so long the vehicle is at high speed in the movement....means rolls or fly's...your body is not creating a big mass
a very good example is Stainless Birthday party 2013.....due to this that the streamliner was tumbling over and over...the force on Stainless stayed low...if the streamliner had a longer stop contact on the ground than it could be serious for him.
now to the weight of the vehicle "slowing" down at high speed.....a heavy weight need longer to slow down....at the salt there is nothing we can crash in (except the salt...)...that means in the other hand....the vehicle needs longer to come to the rest......for your body it means more time to decelerate....but this counts only if the vehicle change not his direction or comes to a sudden stop.....means, when the racer stand on his wheels and spin....or pencil rolls into the moving direction.....
all changes from the moving direction into a other direction creates a deceleration effect on your body.....
it will be always a matter of how the racer crash.....and depends on that....have the luck that he does it the right way
If a lighter vehicle is "easier" on things they run into then it has the same affect on being easier on those inside the vehicle.
A heavy vehicle at 100 mph hits, say, a house. It goes through 3 walls before the coming to a stop. A lighter vehicle just breaks through the outer wall before coming to a stop. All other things being equal, which vehicle has subjected the driver to higher acceleration?