Gearbox

Hi, I am running a Mazda SL3500 Ti with Iveco large intercooler electric fan married to a R380 gearbox and 1.222 transfer box and seem to drive everywhere even with heavy trailer in 5th sometimes having to get into 4th. I will soon be fitting a 14D 1.003 transfer box ( as I have one) BUT am slightly concerned about the standard gearbox I have been told about a "Santander" or alike ? box does anyone have a five speed extra strong box or any suggestions .... will be away for a week but will look at comments on return thanks Pete .

do you have gear box oil cooled ,lt85 santana is what you refer to .but lt85 lr version was the heavy duty one ,the v8 4 ton armoured ran lt85s but switched to r380s oil cooled which worked fine ,L suffix r380 is the best out of them