thanks Jan, that Armorama link is awesome!
Dan, if you want to build the tank carrying a Fascine and want to be accurate, you'll need to convert the whole thing to a Mk IV. There's a good load of reference in landships website here:
http://www.landships.info/landships/tank_articles.html#Short of counting rivets, the main things you'll need to do is the add the rails, remove the muffler like thingy on the rear hull and add a box like fairing. Also add a long muffler pipe from the top running down to the back/rear, and a square enclosure on the roof for stowage.
My rails were made from plastruct L shaped beams. Evergreen has similar products. My rails seem to be installed the wrong way, as the hollow of the L should face inwards and not outwards as in my photos.
Awesome diorama by Roger Hurkmans here:
http://www.twenot-forums.nl/viewtopic.php?f=3&t=26183Otherwise, the Armorama link Jan provided does a really good job on showing what needs to be done to make the earlier marks.
cheers,
Sonny