whether original 4speed gearbox can be modified to 5speed? without having to replace the original gearbox

You need to swap in a K50 box.

Edited July 31, 201313 yr by B.L.Z.BUB

You need to swap in a K50 box.

 

because the 5th gear is outside the gearbox in the tailshaft housing, and the idler gear shaft has to be longer to go through the rear wall and carry it.

 

They are different in the belhousing front plate that carries the thrust bearing too, now I think about it.

 

Just swap them, as Reed said-

I'm looking at doing the same swap (k40 to k50 box)thing tell me if it works, is there a link with directions I have the Gregory's but pics help too

Can 7k gearbox direct fit to 4k engines