A trading Beluga? Well, I never had that idea before. But I was doing passenger missions in 2.2 and 2.3 to and switched in 2.4 from the Luxury-Beluga to an Orca for VIP-transport. That´s not really the same thing, but it´s ok.
And last week I tried the first Sol-Colonia Flight in the just Slidly modified Luxury-Beluga (4 Business instead of 4a FS And 6A FS instead of 6 First Class/28,5LY jumprange) and that worked fine to. Although it took three days i earned about 240mio.
I´m quite curios about the next flight in a more specialized Deep-Space Beluga(~38LY, couldn´t find Arsenic, LUL).
To come back to your intensional Question: Unshielded you maybe get about 368T Cargo, but do you really want to fly an unshieldet Beluga? Since you always loose one of the 6.internals, even if you only fit a 5d shield, you end up with nearly the same Cargo-Capacity as a unshieldet T7 which is easier to fly (especially in Dock) and can avoid interdictions nearly so good as the Beluga, I would recommend the T7. Before the passenger missions I did that with, if I´m right, about 500~600k per travel. So it should be nearly the same in a Shieldet Beluga, if you really want to fly one.
I hope this could be usefull. o7