Scania 420 vs 360

It would be easier to put a extra axle on the trailer then put a extra on the truck.
Cheaper to.

 

Personally, I would gear lower than a 3:42 for the job you described.
The power band on that Cummins was between 1700 and 2100 rpm, much different than modern engines.

In direct gear, and assuming your tire size is 22.5, I would think a gear ratio of 3:90 or 4:11, (maybe even a 4:33), should put you near 1500 rpm at 70 km/h.
Best to use a road speed calculator to give you a better idea, take the guessing out of it.

 

here it is impossible to find parts for 3series for more than 10years.
4series almost dont exist, saw 3 on monday while traveling 650km, which is highly unusual. you can still see early r series due manual tachograph (till 1.1.2009).

113 can have differential r770 or r780, they look almost same but 770 is much weaker.
scania without retarder is useless, here 90+% scania have retarder, even if it is distribution truck without trailer.
well i would select 124 420, be careful, if it have HPI injection you will need mechanics with good skills, you can recognize it by red color inside 0 of 420 label.

 

for this kind of conversion lightest reflect only to front end of truck since he will get chassis and axles from some other trucks. newer trucks are always heavier on front end.
even lightest scania models are stronger than 80% of USA specs trucks.

 

if you are looking to save weight stay away from 4axle truck, and go instead on 3+4.
truck steer axle weights about 1100kg incl steering. trailer axles are bit lighter (depends how strong axles do you need and single vs. dual tires). lightest possible axles we have on trailers weight 550kg including axle brackets, suspension, brakes, air tank, wheels, tires, mudguards. most trailers will have axles with around 700kg

 

that is very difficult to say, under normal conditions 113 360 would make about 35L in 40t traffic.

 

if 113 360 have r770 diff stay away from it by any means, they usually have 3.5 ratio 3.25 / 3.89 / 4.25 is possible too. r780 is released on very late 113, it will be 3.08/3.4/3.8/4.25 and it is more durable but still very marginal for your needs especially after so many years..

back than engine 113 was very light, low compression ratio, would change rpm very easy, is uncontrolable while starting, very high rating of turbo on this engine, so once turbo start to fill it become violent, on offroad truck result with lost traction, after you get traction again you will easy break something.
10speed gearboxes had small overall rating so starting with such engine and fast gearbox result with serious problems.
124 420 is way better for starting under such weights. is 4series 360 114 or 124?