How Important is 'Original'

I have a 64 Chevelle Malibu SS, purchased originally by a family member.  It
has a 283 with a Powerglide transmission.  Years ago I replaced the exhaust
manifolds with headers (manifolds are now rusted inside and out), put a
'chrome kit' on the engine (valve covers and air cleaner... but still have the
original parts), and put on dual exhaust.  However, I had to replace the front
bumper, grill, headlight trim... and will need to replace at least one interior
door panel along with some other interior parts.  I'm debating between
rebuilding the engine (burns oil... bad valves or rings) and live with mediocre
power and a two-speed/whiny transmission, or buying a pre-built 350 and
three speed transmission to put more muscle into it.  I also plan to upgrade
the brakes to disc for safety (at least on the front), so based on what I've
outlined, is there a compelling reason to go rebuild vs. upgrade on the
engine/transmission?