number three body mount help please!
You being well versed in metal fabrication is in your best favor right now. So cutting out teh rot and getting to good parent metal might be in your best inereest..as long as teh rust inside the mounting areas is not so bad to be able to set into to weld to something worth welding to. You may need to fabricat a "pocket" with a lower palte attached so you can weld teh lower plate to teh door sill framework and teh pocket will go up into teh mounting areas and ba able to be welded there.
The problem lies in gettiing the fiberglass to hot and burning..depending on your welding area and how hot you get it during the welding process. That in conjuction with the fiberglass panel (s) that may keep you from getting a good weld point.
Just take you time and be aware of any fiberglass that is against the metal that you are welding to.
"DUB"
Sent you a PM about a local shop that has performed this repair many times. They can probably provide some good advice and are currently doing this repair on a '69 and a '75.
Good luck... GUSTO
http://www.parts123.com/parts123/yb....Z5Z5Z50000050G










