I might be able to help, Ter, as I am converting a 250/9 to a 250/7 Mortar carrier right now. I am having trouble with the left hand side interior of a 250/7, but I have pictures of both for a 250/6, as shown here: http://afvinteriors.hobbyvista.com/250/250c.html
250/7's carried mortar rounds and look quite different to these pics. I will try to chase up a photo to post for you.
FSM published an article on making a mortar carrier by Bob Skurda, but I have lost it, unfotunately. I have seen conversion kits also, but they were too pricey for me, and I like mucking around from scratch anyway.
If you want to know anything else specific, I will do my best to help. I am hoping to post pics of my build in the future.
Matt
There