On my 70 Convertible since I got the car years ago there has been a small crack in the frame on the passenger side over the rear, exactly at a body bolt mount. The crack spans approc 1" on the top of the frame medially to laterally and then down approx 1.5" from the top. I drove the car for years watching the crack and it never got a bit worse. Car has been sitting in the garage for the past 5 years or so and plan on resurection as my daily driver this summer. Will be adding a new dual plane intake, new 600 cfm carb, new cam , ditching the stock ex manifolds for headers and increasing the dual system from 1 3/4" to 2 1/2" with some flow masters. With the modest amount of increased power I need to fix this. Will being putting on new quarters before it hits the road so while I have the mig out I figure its time to fix it. Any suggestions for the experts here on how to weld it. Should I just wled the existing crack. Or should I weld existing crack and then weld a pice of flat stock over top of crack an the top and lateral surface of the crack. Any suggestions are welcomed?

By no means do I consider myself an expert, but from what I've read in other posts I think the first thing you should do is drill a small hole at the end of the crack to prevent it from spreading any further. You said it hasn't gotten any worse, but that could change. A small hole for insurance seems like a small price to pay.

There are other posts on fixing frames I will try to find it and post the link.
You can search on the list for frame repair.

here is the other post it is about rust but the same applays in reparing.
http://www.chevelles.com/forums/showthread.php?t=118704&highlight=frame+repair

This page has repairs of a frame.http://www.einstyn.com/