I notice that you haven't yet been able to post a picture of your dilema and that you haven't got an answer to your problem.
I think I understand your problem and suggest that you need to think outside the box, or board, in this case.
Instead of trying to accomplish the answer with one piece of wood gusset, the problem can quite easily be solved using two pieces as follows.
Firstly, cut 2 blanks of 2 X 8 nominal to the identical size, then temporary attach the two pieces together using long wood screws in areas that you will not be cutting into. Put a mark on the inside of the piece to indicate the two inside faces before attaching them together.
Next, cut the arc profile on your band saw on the 4 X 8 nominal blank. Now remove the temporary screws from the gusset. Both pieces will now be matched with the same cutting irregularities and identical profile and are now ready for routing.
Chuck up a 3/4" cove bit with bearing in your table top router and route along your arc on the inside face of the half gusset. Route the two pieces in a least 3 passes to the overall height of 3/4 +/- " height .
When routing is completed for both pieces, simply screw or bolt the two halves together and you should have the concaved arch profile that you are looking for.
The resulting gusset will be 3 +/-" thick and have a 3/4 "side leg, that will provide lateral support to the gusset.
I am assuming you will be using U bolts or pipe hangers to attach the gussets to your metal framework.
I hope this simple solution is helpful and won't require you to pull out any more hair or lose any more sleep over your dilema and allow you to get on with your project.