I think the the High velocity guns sounds good but do you really want to use old tanks as tank hunters? If you do how about squeeze bore or high-low pressure guns?
Canales light tank, Republic of Rio Grande c.1953
If we use the 8cm PAW-600 as a template...... the potential of being able to borrow manufacturing tooling related to mortars (basis for projectiles; the barrel is smoothbore, too) sounds pretty attractive as well.
What's the typical combat range of late-war standard tank gun (as in KwK-40, M1A1, or ZiS-S-53) in anti-tank role?
I heard that there was a plan to fit a modified Pz.IV turret armed with the PAW-600 onto the Pz.38(t), which is just like if not along the same line as your Canales light tank...... I wonder if the gun can be fitted into smaller turrets...... like that of the Pz.III......
A common gun or at leat ammo is probably desirable, how about the Kwk 39 (or go nuts and put in the BK 5)?
I am inclined to stick with the KwK-39 for smaller tanks