Rear frame rails are very common problem is classic mustangs. Frame rail replacements are readily available at any Mustang restoration supplier.
You can weld it in yourself, or if you know someone who is handy with a welder, use them. When you do start cutting, look at every piece of the frame and any area that may be affected. Look at front frame rails, radiator supports, cowl vents, floor pans, etc.
Try Mustangs Unlimited or NPD
Mustang Unlimited NPDLink | National Parts Depot
Do some research and see where the closest one is for the best price. I don't know where Avon, IN is but I live in crawford county Indiana and I use Virginia Classic Mustangs and Mustangs unlimited.
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"yeah though I walk through the valley of the shadow of rice, I will fear no turbo for torque art with me. Thy rods and thy crankshaft, they comfort me."
1965 Fastback 289 hi-po 3sp.
1973 coupe 302 4V auto. -SOLD
1994 GT 'Vert