I just replaced the black tank in my 1972 Sovereign
Center bath. Yours probably isn't a center bath, but if you have all the parts at the start of the project it didn't take that long. I think I would have been able to do it in about 4 hours if I had to do it again, and knew what parts I needed before I started.
If you do have a center bath I highly suggest you build a box to support the toilet like I did. I copied the idea from someone else on the forum. otherwise the tank supports the toilet and it will be cracked on top as well as the bottom. it's also a good time to switch to a Valterra valve so you can buy a common slinky and not have to use the fragile adapter.
Inca Tanks has the tanks you should need, and I thought it was reasonable for a tank they don't make very often.
I also changed out the toilet for a Dometic 310 at the time, I like it so far, but it sits a bit higher.