Be aware that there are local and National codes that determine how and what you can do. It's not just a question of being handy. It's a statement of safety
A small cylinder (40 lb.) will not give you the service you need.
Usually the local LP companies such as Ferrell or Suburban will do the install for free provided you sign a contract with them for the LP gas.
My son worked for several LP/Natural companies over the years. That was the policy for all of them. Gas companies make their money off of the sale of gas.
If your husband is going to do the work, he has to take particular care with setting the orifices. Natural gas uses larger orifices than LP. Natural gas is much lighter than LP.