Good God Ken! You plan on doing an all sky mosaic in narrowband with a 300mm lens. Certainly a first. I've seen a few 50mm lens all sky mosaics in both lum and Ha. They look great, but at such a scale lack detail which is a little disappointing in my opinion. 300mm would look spectacular. You could always give a hybrid composite a go i.e. use a 50mm for the wide sweeping FOV, the add data from the 300mm on specific targets to give depth. Maybe when you're feeling comfortable with such a concept, give it a try.
As you've found (and can be seen), you've matched the Ha panels exceptionally well. In general, matching the luminance data is quite easy providing its gradient free. Just keep an eye on the stellar profile sizes. If you're stretching the data too hard to match another frame, stars will bloat and not match surrounding frames. You may find that lightly pre-stretching the data using DDP or similar prior to stitching will assist. The trick here is to keep the background and midtone difference value equal between the panels. You'll do less work in PS in making them match, but the use of curves is unavoidable (and in some cases the brightness/contrast tool). Registar has a features to match the brightness of mosaic frames, but I've found it erroneous. Really, PS is the tool of choice. Dennis
posted a link to download some online photography mags that contains excellent information. The first few editions are gold. Having completed numerous intermediate and advanced PS courses, I've not come across this information as articulated in these mags - simple and to the point. The curves info explains the fundamental of the histogram, along with the basic curves process, followed by advanced multi-anchor, then finishing off with the traditional s-curve. The second edition has info on removing colour casts. I don't necessarily agree with one of the approaches used, but the others are good - they'll all work. Anyway, I digress, point is PS curves is the solution.
Colour does add an extra dimension (or should I say challenge). Unfortunately, its all about compromising colour fidelity. There will be some frames that will match relatively well and only require colour balance tweaks in shadows for example, and then there will be others that no matter how hard you try in PS, they wont match up. For these, you're best to performing a colour combine again on the SII, Ha and OIII data again but with different weightings. The replace colour tool can also be handy if you need to tweak hue, lightness and saturation for specific areas.
The approach I would suggest is to stitch your luminance frames first i.e. the Ha data. Match the brightness and contrast, then use layer masks for a smooth transition between the frames. Once complete, build the SII, Ha, OIII combine frames with your usual weights. 4:1:3 ratio is a good start for a SHO palette. I don't really like tools such as MaximDL neutralising the background automatically. I've contributed to a post on narrowband processing using a pixel math method which gives much greater control. It can be found
here. Narrowband ratios are not a strict as traditional RGB so experiment to you get a blend that is appealing to you. Register the colour combined narrowband frames to align the stitched luminance. Leave the stitched luminance frames as normal blend mode and add the colour combined narrowband frames as colour blend mode. You can also drop the opacity of the luminance to regain colour saturation or use other methods such as
colour dodging. Similar to the luminance frames, use layer masks to ease the transition between the frames. Using the luminance frame as the base, you'll address the contrast problem you describe, leaving you to juggle with the colours.
There's no substitute for experience when it comes to mosaics, so keep at it. Keen to see how you progress. Should be quite a spectacular result.
Cheers
Loving it so far.. should be an amazing product once you complete the project.
, 
