Depends on whether it's an early 58 'high roof' or not.
These early ones are worth more if kept original.
Back in the day I had a 57 model and it was a great car, though as mentioned wheel alignment was a monthly exercise and gearbox repair was bi-annual.
The way to tell them apart (other than it having a taller roof line) is the different grille, dash and trim below the tail lights.
The grille has round parkers, the dash is a half circle rather than the later wide ones and below the tail lights was just a straight narrow chrome strip.
Here's a pic from long ago of my old one for reference.
For me personally I think the GM modification would devalue the car anyway, most enthusiasts would be put off by that.
If I was to do one it would probably be a barra or even an AU six.
I would actually love to try one with an ecoboost and 6 speed auto, but that's just me.
Some pics would be nice, still a real soft spot for these old girls.
Cheers,