Weight wise (from navweps)
5' twin Mark 29 Mod 0 (from Atlanta class) = (49,000 kg)
6' twin depends on type (what is yours ?),
- Swedish 15.2 cm/55 (6") Model 1930 = 72t
- old USN 6"/53 (15.2 cm) = 52.6 mt (very old design)
- Britain 6"/50 (15.2 cm) BL Mark XXIII Mark XXI = 91 tons (92 mt) (heavy due to firing fast at 60deg elevation
)
- Britain Nelson (6): Mark XVIII Weight = (76,204 kg)
Not sure what yours will weight maybe 70t ?
So (4 x 70t) = 280t v (5 x 50t)= 250t,
That's 30t lighter for the 5' guns when you add in removing the 3' or 88 ? (you only need to save another 20t)
I think you can go with 6 twin 5'/38 (maybe have 1 each side ? or can you fit another one out front ? That or fit a load more 40mm, probably much more effective till you get VT shells
)