engine pinging / clattering

Hello, I'm from San Antonio, Tx. and I'm new to this forum... I'd appreciate anyones help with a problem that just developed with my 91 5.0 LX Convertible. I've been noticing a slight pinging uder accelleration for a little while and just thought I might have gotten some cheap gas. I picked my car up fropm the body shop today and it's running terrible. Low power and clatters alot under accelleration. I changed the fuel filter, plugs, wires, dist cap, rotor, and check the timing. Runs a little better, but not much. Any thoughts as to what my problem might be or what I should check next?

Thanks,
satx91lx:wavey