Can you alter the height of breast bar in ifor williams trailer?

[Whitegrenache]

Just as title suggests really....
Just become proud owner of 2nd hand 505 and we intend to use this solely for my 14.1 and dds 11.1 ponies....the breast bar heights need to be dropped considerably for dds pony and it looks mechanically doable (dp is very handy), but just wondered if Anyone had ever done this and what do you do with the gaps left behind in the walls when you move positions?

I have googled and there is very few, if any suggestions!

Thanks