Does anyone know if a 1978 5 speed from a 620 truck is a direct bolt in to a 240?

The 620 used the L16, L18 and L20. They have the same bolt patern and bolt position as the L28. It will bolt in, but first gear will be very low. 

I think the 620 has a 1st : 3.592:1

280Z 1st : 3.321:1

280ZX 1st : 3.062: 1

Can the gear ratio be calculated by turning the tail shaft and counting the input shaft?

Did you get one to try? They are geared pretty tall.

Here is some more info on the different transmissions. 

http://datsunworld.blogspot.com/2016/03/datsun-l-series-transmission.html

28 minutes ago, Terrapin Z said:

Did you get one to try? They are geared pretty tall.

Here is some more info on the different transmissions. 

http://datsunworld.blogspot.com/2016/03/datsun-l-series-transmission.html

Picked one up today. Haven't verified  the gear ratio yet. I need someone the help count turns, a mirror doesn't work. It looks good from what I can tell and is suppose to have new bearings and seal and internals are suppose to be excellent. As the leader of our country would say "We'll see", But it's a gamble though. 

 

Looks like the 620 has the 240Z ratios, doesn't it?  Better measure the length.  It might be shorter.

Leader...that's a good one.  Just  'cause you're in the driver's seat doesn't mean you know where you're going.  Especially with a whole car full of backseat drivers.

Forgot to say, you could put a little indicator on the tailshaft that's easy to see.  Turn the input shaft 3.6 turns and you should see one rev on the back.

Yes.  Someone suggested put on an old clutch disc for a more accurate count and turn 20 times. 

I was going to mention that you don't have to be exact on the rotations.  For example, in first gear you'll have 3.592, 3.321, or 3.062, for example.  3.592 is between 3 1/2 and 3 3/4,  3.321 is between 3 1/4 and 3 1/2, 3.062 is almost right on 3.0, for one complete turn of the output shaft.  Set your tail shaft indicator straight up, estimate the turns to get a full rotation and see what you get.  The 1/4 turns are easy to eyeball.  Once you look at the possibilities and try a few counts you'll say "it has to be ____".

Well it looks like this trans pretty much a standard 78 280z. Still acceptable for the price. Next step is to put it in and see what happens. Probably wait for fall so I pull off my stock down pipe customized to connect to the Zstory exhaust and have it ceramic coat it.