I’ve been doing a bit more ferreting for info on these beasts. It seems that the 6x4 RHD and LHD options fell roughly into two categories:
the heavy-haulage tank transporter
and the general haulage version.
Both had the Rolls Royce Eagle 305 bhp lump. The heavy-duty version had a Fuller 15-sp and the general haulage version had the Fuller 9-sp. The general haulage version had a day cab as standard but was available with a sleeper. As mentioned you could have RHD or LHD. Here are a couple of bits of ‘evidence’ (I’ve posted the Russian example before).
And as a footnote, I would add that early (and Oz) RHD versions could have Detroit 8v71 plus 9-sp Fuller 
There are also rumours of a TL12 spec version.Obviously ‘some’ would like to think it would have been superior to the RR.Obviously ‘others’ disagreed.
I think it was down to BRS, CF. The 6x4 Detroit heavy had been in production for some time before BRS did a deal with Scammell and Rolls Royce to produce a 4x2 general haulage 32-tonner to their own spec. Were it not for BRS we might never have had the 4x2. They tried variously the 220, the 280, the 290 and the later 265 RR Eagle engines. Factors such as economy of scale, availability, ease of in-house maintenance, MPG, industrial relations etc etc would have all come into it one way or another. Follow the money, is usually the best lead.
Passed my test at 17 years old in a LHD Crusader in the wonderful town of Fallingbostel in Germany. Happy days, just wish I could remember them 
Salut! If I had to retake my driving test now at the age of almost 73, I’d more than gladly take it on your LHD Crusader! Happy New Year old mate.
Hmm, could I have some practice on the Scammell before the test please? I’m totally confident but have never driven a LHD, adjusting to hanging off a 15 speed with the right hand, instead of the left, might take some re-education.