Can I use the old holes in the cross member or need to drill new?
Sure if U can figure a way to predrill the wood for accurate alignment and just to be on the safe side for good holding power use self tapping one size bigger.
TIP 4 U
Do as they did in aviation to prevent rusting of tubing,
Pour some oil into any tubing and that it will deter internal rust. It works!
Naturally the tubing ends need to be capped,
On a project (my DIY tractor cab) I simply shaped some wooden plugs and hammered them in after I had added some oil.
Now to trailer floors.
I have a friend that has restored many trailers as a hobby/pastime for pocket money.
He collects old heating oil tanks, cuts the ends off and flattens the cut oval by driving his tractor over a few times and coaxes onto the the trailer frame and welds everything up.
He must have recycled some 30 to date and sells them on the net.
There is a big demand for small utility trailers at affordable prices.
I could not believe how some 'wrecks' could end up as very usable handy trailers.
Naturally a new coat of quickly applied paint helps.