I have just gotten a 75 DT250 with a 400 engine in it. It has had the oil pump taken off. I want to put one back on. I'm looking for a pump and all the gear and such that goes with it. My 1st question is will a 250 pump work on the 400 motor?  The numbers seem to be similar for the oil pumps with a few numbers or letters at the end. Are they all the same for that era of bikes...74-77. ?   

They are all similar. I would feel better about running a 360/400 pump than a 250 unit. In a pinch though, especially on a stock engine ridden conservatively, I bet the 250 pump would be fine.

These things seem over-lubed by a fair margin. I pulled some shims out of my 360 pump to shorten the initial stroke and it's been doing just fine, despite the pipe, porting and big carb. 

Appreciate the idea and I will feel better when I can get my hands on a pump and the rest of the stuff to make it all work. Its not easy these days to find it all.

DEET

I'm pretty sure you'll find the helical gear on the pump drive shaft will be different between 250's and 400's .
There will be a matching gear within the pump . They don't mix and match .

I believe I have the right shaft from the motor to the pump. It was inside the case of a 400 motor that I got. I really need the pump and lines and cables and such. I have a tank and such.

They are not all the same.  Critical that you match driveshaft to pump as was mentioned.  So no, a 250 pump won't work with 400 shaft (if that's what you have).  You need a "matched set" at the very least and if using pump from another model amount of oil metered probably will be different.  

DEET might be able to fix you up with what you need.

The shaft should remain in the pump.  It can be pulled straight out but it's not likely to just fall out.  Unless somebody just slapped some mix and match parts together, whatever pump you find should come with the correct worm drive shaft.

If you are unsure of the combination you have I bet you'll find out pretty easily if you turn it by hand.  If the gear and shaft don't match it will bind up.  They should turn with only slight resistance, in operation the pump drive shaft turns counterclockwise when looking at it from the end that mates to the crankshaft.

 Here is a set up on eBay for a 1974 DT360A (not mine) not sure if they are the same, it’s at a reasonable price and may get you started on what you need, and possibly sell the clutch cover that you don’t need to re-coup some of the price?
 
 

Oil pump drive shaft is the same for DT 360 A and DT 400 B , but the pumps have different numbers .
That may only mean , they perhaps use different pulleys .