The biggest issue is this:



This is where he screwed up the frame:

The two angled pieces that were originally welded onto the spine a few inches higher, blocked the tank from sitting down on the frame all the way.
He could have hammered in the bottom of the tank slightly to make room for these rails and left the frame entirely alone, but NOOOO...

He cut the bends out, moved them down, and welded them back on lower. Then realized that the big bell on the top of the stock CV carb was made to fit right in between those bends! so he hammered in half of the frame tube to make room. You can see the indentation on the left side of the frame. On the right side of the frame, he actually started to hammer the tube in the WRONG WAY!, then stopped, flipped it over, and hammered the rest in; so on the right side, there's an indentation on the TOP of the tube (which you can see in the photo) as well as on the inside of the tube.
Essentially, there's only about a third of this tube left in tact, and these indentations are hammered in exactly where the frame is otherwise unsupported. It's going to collapse right there if it gets any stress at all.
Additionally, even with these indentations, the carb just BARELY fits, and is sure to rattle against the frame if there's even slight vibration from the engine. (I'm guessing a nice big 650 single is gonna vibrate just a tiny bit!)

I bit the bullet, and ordered a VM36 from LANCER, which doesn't have the big diaphragm bell on the top, so it will give me a lot more space to get in there and brace this up. The carb should be here next week sometime, and I'm waiting until it arrives to see how exactly it all fits in before I tackle this at all.
I have a few possible thoughts, and I'm willing to let it be a little ugly since it will all be mostly hidden anyway, but if you have any suggestions I'd love to hear them!

Right now, I am leaning towards using some very solid "U" channel, and bending it to fit inside of these frame rails, then welding that into place along the top and bottom. I may add additional bracing from there... who knows.

I'm half-tempted to get a new frame and start over from scratch!  

Beautiful, man, just beautiful!! Simple, clean, and I even like the color. Chrome fenders are sweeeet!

Two questions.  
      Where'd ya get the tank?

      Could you post a close up pic of your front sprocket cover?

Hope you get'er on the road soon! Should be a blast!
   

that looks amazing!

Oh dude you aint dead at all - you are creating something beautiful.
I followed your thread on DTT and wondered jealously where this build was going - I LOVE the tank and the whole look.
I went thru a similar experience with my XS650 but mainly due to all the horrible things that the PO had done to the frame (buyer beware!!!).
I'd say the best thing to do in your situation is get rid off all that botched welded pipe completely and redo them. So basically cut it out with a grinder, get some new pipe for the new frame bits and get it welded. Then you don't have nagging worries that the frame still has issues.
Make sure the new welder uses a smaller diameter pipe as a sleave/support inside any joins and takes his time - I would make sure I am there in person at the time if possible. Find a motorcycle guy who has done a fair bit of welding - it's not rocket science and if he is into motorcycles himself he will probably want to see you right.
One last push and you have perhaps the best looking savage I've seen - and my girl can attest to how many pics there are of savages on our laptop - a bloody lot