Yea the 77/44 is kinda a niche gun. Intended as a short/handy "bush gun" for close range deer hunting. Hence the limited round capacity and lack of aftermarket support. It is also a low production gun.
44 mag bolt actions are not a popular thing. Most people who want a 44 mag carbine buy a lever gun. Higher capacity and faster to operate.
So pretty much you are going to be on your own trying to make what you want.
Having said all that, I just bought one myself. (has not shipped yet). It is a older used one someone has already cut the barrel down to 16 inch and threaded it.
I was wanting something 44 mag, threaded for suppressor use, manually operated (bolt or lever), but here is the kicker, I specifically wanted a wood stock!
That severely limited my options, as factory threaded wood stock lever guns do not seem to be made.
New production 77/44's with the factory threaded barrels are rare as hens teeth and $1300+
So my options were buy a henry x model and buy a wood stock set for it (big$$), or buy a used 77/44 and have it threaded. I lucked out and found one already threaded for $950.