msycamore said:

Should be quite easy to check exactly what shot was added to the 35mm cut after a little closer comparison, nice. :)

The distant roar of the wampa (where we see his shadow lurking) matches on both 70mm and 35mm. If one then ties the music cue where Luke sinks back down after trying to free his frozen boots and then tries to reach for his light sabre, all that - and subsequent sounds - match.

So the extra 5+ seconds occur as Luke comes to and reaches up to find his feet frozen into the ice of the ceiling. The added shot is probably the closeup of him just before he reaches up towards his boots, as seen in the master shot.