There are several versions of the 450 Alaskan in circulation as it was a wildcat that was never standardised and there’s no copyright on the use of ‘Alaskan’. If you go down this route make sure the loading dies match the chamber reamer specs.
There is really not much between a 450 Alaskan and a 45/90, perhaps 50fps, if that. The 45/90 is the more practical alternative IMO. Cases and dies are inexpensive and readily available. Winchester periodically make special runs of fast twist 45/90’s. About 12 years ago I had a Browning 71 converted to 45/90. I had it built for an upcoming buffalo hunt in the Northern Territory. Loaded with heavy cast pills it performed well.
In hindsight by using bore riding cast bullets, you can load 45/70 to 45/90 length and performance, which to my mind gives the 45/70 a huge advantage in flexibility.
As mentioned, you may need to make some mods to the loading area and look at ways to mitigate the effect of recoil on the magazine tube assembly. I consider the wrap around tubing that Johnson used a major eyesore and blight on what is an otherwise good looking levergun. I believe Turnbull simply silver solders the forend tip tenon to the barrel. In my conversion I went a little further and had a Marlin magazine tube stud dovetailed and silver soldered to the barrel. This has proven effective and is totally unobtrusive.