chujiten: "a sword; a katana"

daijr たち 1）長大な刀剣を総称していう。
だい-とう（１）大きな刀。太刀(タチ)。

nipp: 日本刀の一様式。

daijs: [補説]本来は平安中期ごろまでの直刀(ちょくとう)は「大刀」、それ以降の反り刀は「太刀」と書いて区別する。

koj's たち entry only has 1 sense: 人などを断ち切るのに用いる細長い刃物。...

世界大百科事典 だいとう ... 横刀とも書く。
their 青竜刀　entry:
奈良時代から平安時代の初期には大刀または横刀と書いて〈たち〉と読ませ

Based on the dictionaries, it seems obvious that both たち and だいとう refer to large swords first and foremost.
"sword that is long" is not an improvement over "long sword" - if the goal is to avoid confusion with "longsword", we can do that by 
adding more glosses.
I suggest merging senses 1 and 2 into "long sword (esp. tachi)", unless somebody has a reason why that's a bad idea (daijr and daijs 
have them as separate senses). I'm not sure which details are most pertinent to include there.
I get some hits on google books for 中国 大刀 which seem to match "guandao", but I haven't looked it up in detail.

@@ -15,0 +16,4 @@
+<k_ele>
+<keb>横刀</keb>
+<ke_inf>&oK;</ke_inf>
+</k_ele>
@@ -24,0 +29 @@
+<re_restr>横刀</re_restr>
@@ -30,2 +34,0 @@
-<stagk>太刀</stagk>
-<stagr>たち</stagr>
@@ -33 +36,2 @@
-<gloss>Japanese sword carried edge down, as opposed to the katana, and typically longer</gloss>
+<gloss>long sword (esp. the tachi, worn on the hip edge down by samurai)</gloss>
+<gloss>large sword</gloss>
@@ -40,9 +44 @@
-<gloss>straight single edged Japanese sword, from before the 9th century</gloss>
-</sense>
-<sense>
-<stagk>大刀</stagk>
-<stagr>だいとう</stagr>
-<pos>&n;</pos>
-<gloss>sword that is long</gloss>
-<gloss>daitou</gloss>
-<gloss>daitō</gloss>
+<gloss>straight single-edged Japanese sword (from the mid-Heian period or earlier)</gloss>

>- 大刀/だいとう Senses 3 and 4

The Chinese polearm (glaive, more specifically) called "guandao", cannot, by any stretch of the imagination, be considered the same thing as the Japanese sword-category "daitou". (a long sword. I.e. with an blade-length of at least two shaku [ca 60cm])
Daitou is a category of sword.
Guandao is a glaive.
They are completely different.
If it were up to me, I'd have sense 3 and 4 (arguably 2 as well, perhaps?) as a separate entries. (which is how all other dictionaries do things)
…but that goes against JMdict policy.

Having 大刀 noted as a possible option, for sense 1, sounds fine
…but I don't see how it can be done, under the JMdict framework. (hence why I tagged it as restricted, even though it technically isn't. Sense 2, however, IS restricted to 大刀, though not necessarily in historical texts)
Both in terms of the limitation of the tags and such, and also how terms get clumped into the same entry, where they usually would be separate, in any other dictionary.
(I am, for example, quite annoyed at… Hm? Oh, I see my go-to example, of 治る and 直る has, though only very recently, been separated)

>I like Daijisen's "「断ち」の意". I wonder about that as a note in the たち entry.

I'm quite surprised to hear that, given the strong "entries shouldn't be encyclopedic" attitude here, with an insistence that entries be extremely short, even to the extent that a mere single sentence can still be referred to as "encyclopedic", for some inexplicable reason.

Well, I'm certainly not be opposed to allowing a bit more info in entries
…but even with that, I still find it very surprising (if quite welcome), that you want to add _etymological_ information, into an entry.

https://www.edrdg.org/wiki/index.php/Editorial_policy#Merging_Entries.2FTwo-out-of-three_Rule

This edit has some significant problems in that it is really proposing 3 separate entries:
- 太刀/たち Sense 1
- 大刀/たち Sense 2
- 大刀/だいとう Senses 3 and 4
I agree it's not easy to have a simple, succinct set of entries. The major JEs (GG5, etc.) split on 太刀/たち and 大刀/だいとう, but the kokugos all have 大刀 as well for たち, indicating historical contexts.
Maybe the best way to do this is to have two entries:
-太刀/たち (long) sword, with either 大刀 as "oK" or a note: "also historically 大刀"
- 大刀/だいとう with two senses, xref back to 太刀/たち
I like Daijisen's "「断ち」の意". I wonder about that as a note in the たち entry.

https://ja.wikipedia.org/wiki/太刀
https://ja.wikipedia.org/wiki/大刀
Koujien, Daijirin (not its inaccurate jp-en bit!), Daijisen

Tachi refers to a specific type of Japanese sword. (well, two types, though typically distinguished by kanji …and, quite often, context)
Not to any sword that is long
…and especially not to a longsword, which is a medieval European sword type.

@@ -29,0 +30,2 @@
+<stagk>太刀</stagk>
+<stagr>たち</stagr>
@@ -31 +33,23 @@
-<gloss>long sword</gloss>
+<gloss>Japanese sword carried edge down, as opposed to the katana, and typically longer</gloss>
+</sense>
+<sense>
+<stagk>大刀</stagk>
+<stagr>たち</stagr>
+<pos>&n;</pos>
+<misc>&hist;</misc>
+<gloss>straight single edged Japanese sword, from before the 9th century</gloss>
+</sense>
+<sense>
+<stagk>大刀</stagk>
+<stagr>だいとう</stagr>
+<pos>&n;</pos>
+<gloss>sword that is long</gloss>
+<gloss>daitou</gloss>
+<gloss>daitō</gloss>
+</sense>
+<sense>
+<stagk>大刀</stagk>
+<stagr>だいとう</stagr>
+<pos>&n;</pos>
+<gloss>guandao</gloss>
+<gloss>Chinese glaive</gloss>

i don't see a ref for たいとう

@@ -29,4 +29,0 @@
-<r_ele>
-<reb>たいとう</reb>
-<re_restr>大刀</re_restr>
-</r_ele>