Look at the Core pg 118 stats - the Far trader is basically the same as the Free Trader except the J-2 drive, required PP, fuel & processors and Computer upgrade (bis instead of -2). The 22 extra tons of fuel and above upgrades (9 tons) are taken from Cargo and lowberths.
So,
rust is saying they use those stats - just use a modified deckplan from the Free Trader (so it looks just like a free trader).
FYI:
Here is a deckplan that claims to be a J-2 design (but a quick look doesn't match up - the J drive seems to be 20 tons instead of 15... probably another Traveller edition or typical deckplan swag)