While I (as anyone who does shadetree mechanics or other homemade fabrication) love JB Weld for many things, this wouldn't be a situation where you'd want to use it unless of course you enjoy your gas tank falling off.
I would probably go with some brackets and mechanical fasteners (aka nuts and bolts). Just get some angle and/or bar stock from a hardware store or Home Depot and aside from some drilling and cutting that should cover the worst of it. Most of the angle stock like that is prettty easy to cut if you have a sawzall, chop saw, or worst case a circular saw with an abrasive blade on it. You could of course use a hacksaw if time isn't a concern and it's all you've got to use.
You probably will also want to use something along the lines of U bolts for clamping onto the circular frame crossmembers. For these you could probably get away with just using exhaust type U bolts if they're long enough to fit, but if the option is available to you to get better quality ones that would definitely be the way to go.
