I'm a licensed US Customs broker in the US, here are some things I will suggest. If you decide to go with cargo shipping for the large items, it will have to go through customs formalities in Canada, and in the US upon return. Check to see if the museum works with a shipping company from their side (Canada) that might be able to assist you with this. I'm sure most museums/galleries have had similar situations and they work with shipping companies that specialize with artwork. The 8'x6' item will be bulky and might be costly to ship, since it will most likely volume out and will take space.
Paintings and artwork are duty free, their Harmonized Tariff Schedule in the US or HTS for short is
9701.10.0000 It is pretty much the same for other countries as well. While working with the shipping company, it is very important to tell them that goods will return to origin, US. They might opt to do a
Carnet entry, that will allow goods to travel through several countries without any need for customs intervention for up to one year, or
Temporary Importation Bond, similar to carnet but only limited to one country. The only caveat to this is that the goods being declared on the carnet or temporary import bond can not be sold, if one is, the carnet/TIB will have to be adjusted you will have to pay duties for the item sold, along with a penalty for breaching the carnet/TIB.
My suggestion, if you are not going to sell an item, best to go with carnet or TIB. If there is possibility of a sale then do not do carnet/TIB, and just go through the regular formalities of clearing goods upon arrival into Canada, as well as the return to the US. For the US return the goods can be considered US Goods returned, and should be classified as
9801.00.40 duty free. You will probably need to write some certificate indicating the authenticity of the paintings, painters name, and country of origin (US). This will help the shipping company in their process of clearing goods.
Good luck and enjoy the show.