I used JB Weld successfully for a similar situation.
I think you should not try TIG on the car....especially if you are not familiar with that welding skill and have lots of experience. I sounds like you have to have the crack horizontal so that the puddling of molton metals can be have gravity help rather than just run out when liquid.
I successfully usedJ B Weld to repair a crack in a motorcycle (Yamaha FJ1200) crank case. Alum. crack about 1/2 long which seeped a small amount of oil. Just like someone else reported, It won't stand up to structural stress but has completely stopped the oil for several years now. Clean the area outside the crack with laquer thinner and sand paper. Deep scratch a 1/2 inch "border area" and apply JB (about 1/8 or more of depth)before any new seepage of oil contaminates the area. Try it you don't have any thing much to lose. $4.00 or so! That Tig is going to make you buy a whole new oil pan or worse burn up your car......my 2 cents !!?
good luck man,
Wish I had someone to help me replace the water pump on my 300E.. That sucker is BURIED under a lot of other components!!