Hi Jason,
Easy enough to weld, I have done several, along with whole new head tubes etc, but in 7005.
However, with 6061 the post-weld heat treatment cycle required is hugely more difficult and expensive.
-Clean off any paint or lacquer back to bare clean metal.
-Cut out crack to a v-notch with cutting tools, not grinding.
-Thoroughly clean inside and out, finishing with acetone.
-Weld inside and out with 4043 filler rod in one heat cycle.
-File back as you feel fit, cut out internal weld material with cutter, and ream out to spec.
The frame would need normalising first (1 hr @ 525C), then quenched (Back to room temp @ minimum 10C/sec).
Then re-aligned within 8 hours and immediately artificial aged back to T6 temper (5hrs @175C).
There are UK companies that will do this, but they are used to doing big batches of aerospace work in large batches.
Expect to pay £400+ for a single item, and to do the re-alignment yourself, so this is usually just for the Klein collectors...
Capalex
Alloyheat
Hauck
HT2000
Depending on how much tube length is available, it would probably be more cost effective to have the tube welded as above.
Then shrink fit a stainless ring on the exterior as a reinforcement band like on this Almega:
All the best,
