The 1070 is unrelated to the 990, although the features are very similar and both appear to use the same Cirrus chip (49400). The 17.2" dimension on the 1070 looks to be the width, however - the standard width for an equipment rack. The depth is 15.5", which is about 2.25" shallower than the 990. It's also 5.9" tall, which is close to 2" shorter than the 990's 7.75". My cabinet is probably similar to yours (around 18"), and even with the 950 (14.75" deep) I had to cut into the back to facilitate wiring (due in part to some stiff cables that needed to be able to go straight back a few inches and in part to the need to reach almost every corner of the rear panel). When the 990 arrived, I just cut out a hole the size of the 990 and pushed the rear panel all the way back through the hole.
It's hard to say yet how the 990 and 1070 will compare side by side sonically. Most of the features appear on both, although until we see the 1070 manual we won't know how the interfaces compare (for example, if the 1070 will have the 990's editable input names). The 1070 does offer the benefit of not needing a separate amp, assuming your speakers are not too exceptionally difficult a load to drive - which would work well for moving it to a separate system later. You could also explore 7.1 without buying additional amps.