Can't help with a March allocated one, but it would appear that there were 6 30tonners (of which RS1076/30 was one) which were delivered at the same time and had 6-wheel runners.
More info (and a picture) here:
http://www.rmweb.co.uk/forum/viewtopic.php?f=5&t=2282 The fourth post down says:
"Drawings & details of LMS 30T Cowans, Sheldon & Co. Steam Crane appeared in October 1972 edition of Model Railways... ...by Peter Tatlow.
Numbers for the LMS type:
RS 1066/30 + Match 770011
RS 1072/30 + Match 770006
RS 1073/30 + Match 770010
RS 1074/30 + Match 770007
RS 1075/30 + Match 770009
RS 1076/30 + Match 770008"
And a useful pic here:
http://www.rcts.org.uk/features/mysteryphotos/show.htm?img=66-59-12&serial=6
I've had a thought about your question on the jib runner - I have a suspicion they were probably purpose built as I can't imagine there were many steel-underframed 6-wheel vehicles being withdrawn even in the mid '40s) to provide secondhand underframes.
That should be enough to get you well and truly moving on what would be a very interesting modelling project!
Steph