$300 is very reasonable for making a revolver cylinder in a small shop. But if you have "seen some on GB that had a 44spl cylinder that came with them", wouldn't it have been easier and a lot cheaper to buy one from Uberti's importer or Gun Parts for less than half that?
As for fitting a cylinder, the timing is not usually the problem, headspace and barrel-cylinder gap are. Fitting a SA cylinder is easier than fitting one for a swingout type gun where the crane fitting and ejector have to be taken into account, but even an SA cylinder can be a bit tricky to fit.
Of course, I have no idea what a gunsmith made cylinder would look like; it could be a perfect fit. Or a less than perfect fit.