@ Britman:
I had the same problem when I bought my Record AC 400.
The fixing kit came with chains which if used at full length would have meant I would have had to almost go on hands and knees (!) to get under it in my cellar shop. There was also a bit in the Manual about not mounting the unit within "X" of the ceiling (I forget what "X" is off hand, sorry).
Anyway I contacted Record and explained my problem and the bloke there said provided there was plenty of air flow all round the sides of the unit - i.e. not tight into a corner or hard up against a side wall) all should be OK.
So I made up four brackets (just MS strips) with a kink in the middle of each so that when mounted on the ceiling the top of the unit stands about an inch (25 mm) below the ceiling. And I chose a place right in the middle of the shop (so no air flow interference from corners, walls, etc). But note that you DO need that "about an inch" as above, otherwise you won't be able to open the two clips that retain the filter - they rotate upwards.
That was a few years back now and it all works just as advertised., and I'm very satisfied with the unit
Based on all that I'd agree, with the mounting very close to the ceiling will be OK, but NOT if you also want it tight into a corner. I think it's either one or the other, sorry.
And the unit is rectangular, so the filters only go in one way, sorry again. And note the retaining clips, mentioned above.
But HTH anyway.
Edit for a P.S: Sorry, I mis-read your post Britman - I thought you meant turning just the FILTER sideways 90 degrees, not the whole unit. I'd say the following - the fan in the unit (it's a drum type) runs with it's axle parallel to the "normal top" of the unit (if you get what I mean). That fan runs in a bearing at each end of the fan's "axle" (i.e. one bearing at each side of the unit). At a GUESS I'd say that turning the whole unit on its side would NOT be a good idea as the the fan axle bearing at the lower end would then be carrying all the weight and forces of the whole fan on it's own. Doesn't sound a good solution to me, but as said I'm GUESSING. Why not contact Record and ask? I found their web site and the necessity to open an account a bit annoying, but once through all that hoo-haah the bloke I ended up speaking to seemed pretty knowledgeable and very helpful.