On your deal you need to decide on how much more HP and torque you want as well as the budget
Being a C body convertible I wouldn't want to risk tearing up the original numbers matching block by hopping it up
If you where my customer I would advise you to find a good 400 junk yard motor that had no holes in the block or cracks in it and use it to build good pump gas 505 to 512 C.I. low deck motor with a good set of new aluminum high flowing heads, intake manifold, bigger carb, cam and kit and the other parts you want to use like a good set of headers and larger exhaust system from the headers back, if your good with using headers
The only bad things about doing a motor like that like I want you to do is it might scare the BeeJesus out of you the first time you hit the loud peddle to wide open throttle
AKA aside, those motor make a hoop of power
PM me on here if you want more help on this