You didn't say how close to the ceiling the top of the screen will be located. I think you will find that most projectors with a fixed lens offset, i.e., no vertical lens shift adjustment, will not work for your situation as the ceiliing mounted projector must be located well above the top of the screen and with your low ceiling this is not possible. For example in the case for the HD33 with a 120" screen the center of the projector's lens must be about 8.5" above the top of the screen. You will probably need to move up to a projector with a vertical lens shift with enough range to allow you to locate the projector below the top of the screen. For the ceiling beam, does it run parallel to the wall where this screen will be located (this is what I assume, but its not clear from your post)? Assuming it is parallel to the screen and located between the projector and the screen then that means that the highest you can locate the projector will be less the 6.5' height of the beam (e.g. perhaps 5.5" depending on your screen location), as you must have line-of-sight from the projector's lens to the top of the screen. Most budget projectors lack a lens shift adjustment and usually require you to mount the projector above the top of the screen (or below the bottom of the screen if the projector is table mounted). Even the Acer 9500BD which has vertical lens shift, has a very limited range (+/- 12.5% or 25% total range) which given its starting point for lens offset does not appear to work for your case. You may need to get up to the $2,000 to $3,000 price range of to find 3D projectors with a wide range vertical lens shift adjustment to support your requirements.
Blog + Reviews + Articles: projectorreviews.com