Modifying/welding custom cranks
I've recently made the switch to custom 200mm cranks on my daily rider to fit my long legs 'properly' and would like to make the switch on all my bikes. Rather than pay $450 to $600+ per set for low tech square taper cranks that only come in 130bcd, I was thinking of welding extensions onto standard cranks that I pick up dirt cheap on eBay. I discussed this with my uncle who is a supervisor at a machine shop and he thought this would be a good and cost effective way to go about doing this. He doesn't have any bike experience though and as I'm sure some of you framebuilders out there have aluminum welding experience, I'd like to hear your opinions as well.
What would be better:
1. Cutting the pedal threading off, welding on one piece, and drilling/tap a new hole for the pedal threads?
or;
2. Cut the crank in between the spindle and pedal hole, then weld the extension in the middle.
What are the pros and cons of both aspects as far as cost and strength/durability? One weld but with drilling and tapping or 2 welds with no drilling? Are cranks usually 6xxx or 7xxx series aluminum? Does it matter when welding and are cranks sometimes made from different alloys? How many hours of labor per crank do you estimate this would take? What type of general labor rates would I be looking at most likely if I went to a framebuilder? Or would a general welder/machinist with aluminum experience be a better bet?