defintely re-store....
I have 351m/400's in all my trucks w/ no problems...loaded the dump truck may lug a little bit but overall you should be happy w/ it
nos/oem parts are available but can be costly, aftermarket and used are an option
as far as the headers go, had them on my plow truck, they rust out and can be a problem replacing, can overheat the starter if not wrapped or heat shield in place...I ran true duals from manifolds back, no loss of performance, in fact I felt it performed better w/ the low end torque

good luck and welcome to FTE