I'm planning a G.A. flight from South Africa through Mozambique with a Cessna 182. The routing is:

Mpumulanga Int'l to Villanculos to Beira to Gorongosa to Nampula to Lumbo to Ibo Island to Pemba to Blantyre to Harare

Any helpful tips to avoid hassle flying general aviation in Mozambique would be appreciated!

Are you sure you can get the Avgas?

Make absolutely sure there is AVGAS, it is a big problem in Moz!
Take cash! And on the issue of cash:
If you are going to take US dollars, make sure you take lots of small bills (they never have change),
Do not fold the money (if it's folded they don't take it),
and lastly,
Make sure all the US dollars are newer than 2006 (they don't take Dollars older than 2006)!
Otherwise, enjoy some of the most beautiful scenery!

I know that there is probably a reason that your routing goes through there, but you could also go up through Botswana and have a hell of a lot less hassle.
All of the major airports there have AVGAS.