You deffently want the weight of the car on the wheels and you deffently want to weld the subframe connectors in. Bolt-ins will eventually work loose as the body flexes. The Mustang chassis is very flexy...
If its not too late check out Maximum Motorsports full legnth subframe connectors:
http://www.maximummotorsports.com/
Subframes connectors are pretty darn cheap really (and sorely needed), might as well do it right the first time around.