The tailshaft out of the B310 will work in the 1200 along with the 5-speed (either that or one from a B210/120Y). Get the clutch with the transmission too, as the splines are different.

I swapped in the B310(210) 5-spd and did not need a customer tailshaft.

I suppose it is possible different rear u-joints might have been used in different B310s, but if so you should be able to find a stock shaft for it.

For me, having 5th gear out weighs the differences with 1st gear.
I have a pic in the gallery that shows the bracket I used to hold up the back of the 5spd. Its not the best solution but it holds. I think i could have used a modified 210 trans mount, but it would have hit my exhaust. I used the stock drive shaft and your speedo will be wrong (fast).
What else can you use...
the radiator, the front suspension.
I think that's it. I'm not fully awake yet.