You would be running your planks the opposite direction of the cross-members.
I would suggest looking at various trailer manufacturer sites for specifications on how far apart those supports are placed... some manufacturers leave much to be desired in strength and then the majority will be pretty close to each other in building specs.
Your ramp if it is that heavy to lift probably had helper springs to ease the load.
If you have a garage door, the springs look sort of like what you have up in the air...same idea of the help to raise the weight of that door off of you.
On the side of the ramp you would see a tab with a hole in it... and again a tab with a hole in it on the back of the trailer next to the where the ramp closes shut.
Seriously, go look at some horse trailers by your barn or at a horse show...if they have a ramp they have something to help assist with how heavy it is to lift and secure that ramp.
Before you toss that plywood floor make sure your ramp has a strong ramp floor... then make sure there is something on that ramp the is good for traction as you load... rubber mat is common but I have seen other things used that worked ...
Save and reuse what you can... projects like these have a way of getting expensive quick.
Good luck and enjoy your project.