just need to know in my stock 75 b210, im doing a auto 2 manual conversion and have got the 5 speed 56a gearbox from the datto sunny and im not sure if im gonna need a new tailshaft, i cant compare em for a while cause my car is 300km away with a busted radiator and head gasket thanks to my dear brother, any help well apreciated.

Yes, you're going to need a different tailshaft. See Propeller Shafts tech section article.

any idea what tailshaft ill need, i know the auto trans shaft is 15mm longer is it posible to shorten it or should i look for The 56 or 60 series trans shaft as im getting the 56 series gearbox?

You should get a 1975 B210 manual transmission driveshaft. 56 and 60 series boxes are the same length, so the shaft is the same. You might be able to use a B310 driveshaft, it all depends on *which diff* your car has.

does it matter if the driveshaft is from a 4 speed/5 speed manual transmission, is the driveshaft the same in both?

I recommend reading the 5-speed and Propeller Shaft articles. They will answer the question generally speaking.