I have acces to an Arc welder. Can I use it to solder the manifold flange to the tube (reinforce) before porting my exaust manifolds? Heads off, taking advantage of everything I can... Thanks.

Do you want to solder or weld? Not sure since you mention an arc welder, then mention soldering? I don't see the harm of adding reinforcment welds, but there may be expansion/contraction problems if there is too much weld, others would know, I'm not too good on metal. As you probably know, if the existing welds are bad enough to break while doing the porting deal, solder(mostly lead) isn't going to hold them together any better.

I have a small hair line crack on the exhaust manifold thats between the engine and the firewall. Is it safe to put 220v through the body of a car to arc weld it??

welders generally don't put out 220V. more like 35-50 volts at high current.

put the ground clamp on the mainfold or the block. disconnect the battery before working.

to use an arc welder you need very fine rod and low current. as low as the welder goes. even then it will be hard to not blow a hole in it. A MIG or TIG welder is much better on this application.

NO NO NO. If you were like 80 years old and been welding you whole life I would say maybe. But you need a mig or tig to do it. A stick welder will blow holes in it. I once say an old welder weld "not solder" to coke can together.

Filling in the back of the flange where the tubes go in with brazing rod will stop a lot of vibration and stress cracks. Gotta use a torch.

NO again NO. brazing is not much better than soldering. Just weld everything

You can actually cause cracks with brazing. bad ones.

My neighbor who works in the motorpool (mechanic) at the local Air Force base told me I needed a TIG welder to fix the cracks in my manifolds. So he took them in with one of my heads that had a couple of busted off manifold bolts and did them. He got them back to me the next day with cracks welded, the flanges re-welded, and all the obstructions ground out of the ports as well.

Your tax dollars at work.

Perhaps TIG welded manifolds will hold. We have not seen (presumably conventionly) welded manifold cracks last very long before recracking.

http://www.headersbyed.com/
A very informative how to on header design, construction, and why some commercial headers don't seem to help much.

The reason the cracks reappear on most repairs is that the metal has been weakened in the area of the crack,welding will make it even more brittle,and...
I wouldn't recommend a stick weld on something that thin either,unless you're God or something.A bunch of carefully formed patches,of the same thickness as the header material,TIGed in place would be my method of choice.

Fiero manifolds are made of S-409 Stainless Steel. The best way to weld a 4-series Stainless is to temper it when you weld it. If it is tempered and TIG welded properly the joint should be stronger than the material around it.

-Brian