Ok lets work through this...

The MANTA is a manufactured yacht and is sailed as such, so a DIY copy will NOT be allowed to sail with a Manta fleet and actually de-values the class and the sport. The reason why is it will be difficult to sell in the future and who would buy a yacht that can't be sailed with other yachts = NO resale value.
Also remember that although a Land Yacht looks simple a lot of design has gone into the yacht you now see around for example the LLMini yacht (with free plans on this site) is a culmination of the successes and failures of probably 200+ yachts.

If you are going to build a yacht please build one that meets the class structure world wide or your region at least it will retain its value when you come to sell it and either go further into the sport or leave in the future.
As you are in the USA I would suggest you get in touch with NALSA or the clubs in your area, and sail the classes they sail.

But getting to the answer you probably actually want about tube sizing.... A lot depends on the tensile strength of the actual tube you have access to, most aluminium you will find at stores is a low tensile strength and is just not suitable for land yacht construction, aircraft / hang glider tube is much more suitable... BUT it is normally of very light wall and has not a lot of strength.
Thick wall aluminium scaffolding pipe has been used in yacht construction due to the 'alloy grade' but welding it will loose that strength at the weld or drill point.

I guess my intentions should have been explained a little clearer. This project is NOT for any kind of competition or adherence to a design class. I'm just making this for fun. That being said...in my searches, I found the Manta to be the closest to what I was already designing. I have a 55 sq.ft sail with mast and rigging that I have used for an inflatable sailboat conversion. I thought I could put this sail to use on a land sailer and possibly even an iceboat one day. I thought it would be neat to have one sail for water, land and ice.

So back to my initial inquiry. I figure somebody here has a Twinjammer or built something similar that could support the weight of 2 people. I'm just looking for their input as to what grade, size and thickness rear axle I should be shopping for.
Thanks!