Large tank on smaller stand support question

[earhtmother]

I know I've asked something similar before but I can't for the life of me find the original pos & replies - not entirely tech/app/NG savy.
I have a 37L x 13.5W stand built from angle iron that has been holding smaller (30g) tanks for years with no problem. I'd like to put a larger tank on top but as it is 18W I would have 5/8 peice of plywood centered underneath it to support the wider width. Is this going to be ok as *most* of the tank would actually be sitting on the stand with only 2.5" on either side only having the plywood base beneath.
1st pic is stand as it is now with a plywood insert to support current 30g
2nd pic is the bottom shelf just to show how current plywood is actually sitting IN the angle iron

[Autumnsky]

I would seal / use some hydrophobic coating on any plywood edges and wood. Not sure about the weight and spread of weight over the wood and stand to give a good opinion.

As a side note, I would keep looking for the correct size stand to be on the safe side. Even if the other was okay and a decent fix. So many places to find them like the Habitat Restore, Goodwill/Thrift, yard sales and local free ads.