jake i once owned a marlin 45-70 & sold it to fund another gun . kinda sorry i sold it . but moved on. but i do have a cva 45-70 single shot with a leupold scope on it . my consider selling it . i know i my end up buying a single shot 350 legend due to my should injuries. you know how i like the 450 bushmaster. great cartridge.
also if you went with the 350 legend your kids could use also. but again it's not a 45-70. let me know & i'll talk to rob @ the gun store we stopped at & see what he can get you one for .

I was going toss my ideas out there until I reread your opening post and realized they don't fit.
I'll mention them so everyone can chuckle at my lack of reading comprehension.
Marlin 336 in .35 Remington, oops bottleneck cartridge
450 Bushmaster, oops again AR platform

anyway...the 45-70 in a Marlin big bore is very alluring.
If I buy another rifle that what I want.

enjoy,,,i would keep the 99

Jake I'm sure you know my answer to this one. I'm a huge fan of the .45-70. I also handload and except for my Pedersoli 1874 Sharps which shoots the heavier bullets, I've found you don't need any more than the 300 gr Hornady Interlock HP bullets for either deer or bear. I send them out the bore with 51 gr of H4198 and it pounds the snot out of the bear. I've only taken one deer with it because I use other rifles or muzzleloaders if I have a tag left by the time rifle season rolls around. None of the bear I shot with it ever needed a second shot and neither went more than about 30 yards.
Like previously mentioned the .444 uses a pistol bullet. Adequate yes but would not be my first choice. The .450 is a good cartridge but it can't do anything the .45-70 can't do. Either one of these can use up to 500 gr bullets if you handload and cast your own but these heavies are unnecessary.
One other thing you might consider is ammo availability. Now you would likely bring your own handloads on a trip. But you're getting up in age and you could possibly forget to pack it!!
You can always run to a Walmart and pick up a box of .45-70s or even possibly .444s. Not so sure about .450s .
And if you can I'd hang on to the 99. Could be a valuable item soon to a collector.