Which frame is built better, new or 20 year old Trek 520?

I know Trek has kept the 520 frame pretty much the same over the years, but
which is the better built frame, my 20 year old lugged frame or a new 2004
one? I am looking at upgrading the drivetrain on my old bike or getting a
new bike, if I get a new bike, it will be a Trek 520.

I am looking at spending a few hundred dollars to do the upgrades that I
want verses the $1,000 or so for a new bike. I really don't see the need
for a new bike, other than some upgrades, the frame is fine and I just want
to get some of the components out of the 80's and into the modern world.
After the upgrades, my 520 will be functionally close to a new 520, although
it won't look as pretty as a new one.

I am a heavy rider and my Trek does well for me. It may weigh more than
other bikes, but on the century I did last weekend, I passed people on much
lighter bikes and finished before them. As you may have guessed, I care
more about how the bike functions and holds up than how it looks. Is there
any compelling reason to get a new 520?

Thanks for your opinions,

Michael