Not sure exactly how traditional you want to be. You might be able to squeeze in a small ( or very soft) bow tie, or some kind of cravat, (a la Robbie Burns) in that Montrose gap, but McMurdo is absolutely correct about the dart. Depending on what you have in mind, you may have some other options.

I have just looked at four different Montrose jackets. the gap between the collar ends varies slightly from example to example, but seems to almost always be between 2 and 4 inches.

By contrast, the three or four Nehru style jackets I can see handily tend to come close to meeting without actually touching. Their gap is usually around an inch or an inch and a half. Piper's tunics and some other military uniforms actually close, either with a large hook and eye or a hook and an overlapping piece underneath / behind it. Leaving the neck slightly open might work for a certain bohemian effect, depending on how you fill that space.

In addition to traditional "single breasted doublets", you might examine other high necked jackets, like military tunics, parade jackets, Nehru cut tuxedos, etc., to see if one of them will do what you want. You can easily swap buttons out for square Scots style ones. If your conscience doesn't bother you, you might consider a USMC dress tunic, which is dark navy with some very snappy red piping. I do not know if you could use the top pockets or would have to try removing them. There is an eBay seller in the UK who offers velvet Nehru style tux coats and a company called Tripp sells various "goth" parade jackets that might work for you, usually in dark cotton twill with a zipper up the middle. If you have some time, you might also check out band uniforms, like the coatee in my avatar.

Do let us know what you come up with.