with todays oils and fuel you have two areas that are problems----oil for flat tappets and UNleaded fuel killing the valve seats------your best bet for some pep under $1000 would be to do Edelbrock aluminum heads/manifold and maybe a ss shorty header set (under $100 on Ebay)--------if you don't have any cam/lifter related issues you can keep it as long as you keep lifters in same holes/lobe as they are now and use Brad Penn or Joe Gibbs race oil.

An option for folks like you that have one of these era cars that I suggest is to remove the present oem numbers matching engine and replace with an later factory roller lifter block(I'd use Dart SHP 4.180 bore, ), stroker crank, for 427 inches with roller lifters, alum heads( there are some super iron heads out there for spec class racing with hardened seats) ------------you'd be at 427 inches, 450-475 HP