zinsco breaker box in garage
Well, I'd suggest selecting a new subpanel with a lot more than 8 spaces.  For expansion room, mainly; if you get busy you can fill it.  (you can run a LOT of stuff on 50A feeder).
You probably weren't imagining a 24-space panel, but the fact is, panel spaces are laughably cheap... whereas running out of spaces is a major frustration.  So it really, really is &quot;false economy&quot; to scrimp on the size of the panel.  Buy spaces like you never want to buy them again. Because you don't.
The new panel will be taller and you won't have any trouble getting all the cables fully into the panel and 1/4&quot; past the cable clamp while they are still in the sheath.
A new installation would require a separate neutral and ground.   Also, ground (alone) can be retrofitted.  However, simply replacing the panel to get rid of that Zinsco should not oblige you to upgrade to a grounded feed.  Also, adding individual circuits should not have that effect.