Cracking between quarter panel and dutchman panel

[brotow]

Is there a good (easy) fix for preventing cracks between the quarter panel and the dutchman panel?   I noticed my 70 Challenger convertible started cracking on the drivers side, nothing on the passengers side (yet). Would welding a piece of steel under , inside the trunk across the seam, a good solution?  The car has torque boxes, sub frame connectors and front  inner fender and shock stiffener kit.  I'm just happy it's not painted yet.

[1 Wild R/T]

   It was the "Easy" part that requires laughter....

 Been through this many times... Tried many fixes, sometimes on the same car, sometimes on on different cars, total of 11 cars & probably a total of 20 repairs....  Here's a few of them....

Tried welding it solid & regular filler, cracked within a few hours...

Tried welded solid & fiber reinforced (Kitty Hair) type filler, cracked within a month

Tried welded Solid & USC All Metal filler... Cracked within a month

Tried welded solid & cut a piece of 16 gauge steel to fit in the trough & welded it solid on both edges.... Then finished with regular filler.... Minor cracking after a year but better...

Tried welded solid & filled with lead, did it myself & I'm not the master of lead... Lasted a couple years but had minor cracking....

Had it leaded by an old timer who's been doing lead since the 50's.... Been that way for over 20 years & it's perfect....

[brotow]

I found  extremely poor welds. Unbelievable considering the car was owned by a very famous car customizer/builder.  Added a piece of metal  underneath  and welded the cracks.  Keeping my fingers crossed.

[340challconvert]

  It was the "Easy" part that requires laughter....

 Been through this many times... Tried many fixes, sometimes on the same car, sometimes on on different cars, total of 11 cars & probably a total of 20 repairs....  Here's a few of them....

Tried welding it solid & regular filler, cracked within a few hours...

Tried welded solid & fiber reinforced (Kitty Hair) type filler, cracked within a month

Tried welded Solid & USC All Metal filler... Cracked within a month

Tried welded solid & cut a piece of 16 gauge steel to fit in the trough & welded it solid on both edges.... Then finished with regular filler.... Minor cracking after a year but better...

Tried welded solid & filled with lead, did it myself & I'm not the master of lead... Lasted a couple years but had minor cracking....

Had it leaded by an old timer who's been doing lead since the 50's.... Been that way for over 20 years & it's perfect....

You have it right: have the area leaded by someone who knows how to do it and you are done!
I have a crack in same area of my A66 vert. i am looking to do what you recommend.
 

[67vertman]

This is how i did mine, "welded solid & cut a piece of 16 gauge steel to fit in the trough & welded it solid on both edges.... Then finished with regular filler" (I used all metal filler).... No cracking after five years, but I do have sub-frame connectors and torque boxes, which really help with the body flex.