Split Rim Tire Change

[rdauphinais]

Ok so my 72 gmc c5500 i nust picked up has the firestone rh-5 5* slpit rims. (The widowmakers) unfortunately the tires that are on it are like brand new, but they sat for so many years that most of them are cracked on the sidewall. Which up here in nh wont pass inspection. So my question (s) are:

What are my options?
Where would i buy some 22.5's or 19.5's? If i want to spend that much.
Has anyone here changed a tire on this type of split rim before? Is it that difficult?
Forgive me for sounding ignorant, but i have alot of experience with rims and tires, and the split ring type as well, but never touched one of these split "highly dangerous" rims ive heard so much about. I do have a tire cage if that makes a difference.