We have a number of Mustang Performance shops listed in our Mustang Directory that are located in the Mid-Atlantic area.
Mustang Directory Performance Shops - US Mid Atlantic Area
Might be worth checking them out and if nothing else perhaps giving one or two of them a call for a price check or asking them for a shop more closer to you. When it comes to getting sub-frames welded in, one thing is for sure, you get it done right the first time around. If you have to pay a little more to make sure that it is, it may well be worth it.
Hope this is of some help and good luck wth the install.