So I have been experimenting with welding to get cranks any length. Todd Sorlie of Alpha Engineering and Mfg. made me the equivalent of really long pedal-extenders (Knee Savers). I use them to fixture cranks for welding, and to align them, if needed, after welding.
Your other budget options are to use "Euro" type BMX cranks, or drill and tap new pedal holes further down the arm.
View of the simple fixture. Two aluminum bars, and two vice "V" jaws. Easy to adjust for different lengths. Todd's "Pedal Extenders" screw into eye and a straight BB spindle is installed in taper. When these are parallel, crank is aligned.
TIG welding is no problem, once I figured out which filler rod to use. Dave Gray told me that, with a few hi-buck exceptions, cranks are 6000 series aluminum. This means 5356 will work for a filler. Vee'ing the arms for welding penetration takes much longer than the actual welding. I can also adjust the Q-factor a bit.
Half the arms I've done have come out straight, and the others have needed only minor adjustment. Easy to make tiny corrections as the heat affected zone is pretty soft for the first few hours after welding.
Saved 22g per arm by shortening this fat crank arm from 175mm to 153mm.
Weighing left crank arms is interesting (right arms are too heavy for my scale). Really fat, cheap MTB arms are 230-250 grams or so. High tech, hi-buck, Shimano 105 Hollowtech weigh 205, about the same as older cheap road bike cranks (probably much stiffer though). But old Stronglight arms come in at 170g, and the SR copies of Stronglight are 160g.
Strength Issues - I wait a week or so before riding welded cranks, but may look into artificial aging. Mike Wheeler knows heat treating guys and can find out temps and times. Due to liability issues, I don't think I'll be shortening cranks for others anytime soon.