Welcome Maas,
I think the guys here will agree with me, this is the swap for you.
Find a 70-73 240 with as little rust as posible. Buy the JTR kit, $350. It comes with the motor and tranny mounts, the driveshaft yoke to fit your shortened Chevy shaft to the Datsun diff. BTW, like Lone said, swap the R180 in the 240 you find for an R200 out of a later 280 or ZX.
If all you plan to do with it is drag race the TH350 will work fine. But if you plan to drive it on the freeway shell out a couple more bucks for an overdrive tranny. You'll be much happier.
With that motor you'll be in the 12's no problem.