Navweaps gives the following for Flak 30/38:
Gun Length oa 88.7 in (2.2525 m)
Bore Length 51.2 in (1.300 m)
... and this for M621:
Gun Length oa 87 in (2.207 m)
Bore Length @ 57 in (1.460 m)
The Flak 38 would probably be the better choice, as to my eye it looks a bit more 'modern' than the 30.
![icon_fsm :icon_fsm:](https://beyondthesprues.com/Forum/Smileys/clabIcons/icon_fsm.gif)
Tamiya do the Flak 38 Flakvierling in 1/48. So you'd have more guns to play with.
![Wink ;)](https://beyondthesprues.com/Forum/Smileys/clabIcons/wink.gif)