What inline 6 do I have?

So I have a 1970 ford mustang Coupe I'm rebuilding one piece at a time it'sy first project car/ my first car, I'm stuck right now because I don't know what inline 6 engine it has there's the 200 and the 250 the engine is old and I don't know where to look to figure out what engine I have any tips on how to figure this out where I live you need insurance to drive the car and I haven't applied for insurance on it yet since it still doesn't run so I can't drive it to a mechanic for him to tell me thank you

The VIN will tell you which engine it has.

The VIN will tell you which engine it has. That is, unless it has been swapped out for the other. Then there are serial numbers on the engine block under the grease and grime that will indicate which engine it is.
When all else fails, take off the head and measure the diameter of the piston and the stroke it takes down… And that will calculate out the engine size. Then there's the numbers on the carburetor. The little engine gets the little carb and the big engine gets a different carb. Sometimes it is the sticker on the Air Cleaner that gives it away. You would not use a little air cleaner on a big motor… Or vice versa. Maybe it is a Chev block.
Can you say FUNNY CAR? Cuz that would be funny.

Find the vin number, from that you should be able to figure out the engine size

The VIN number will tell you what engine should be there.
As for which engine it is- that is a different issue. The stroke of the two engines is different, so to know which one you have would take measuring the stroke. There should also be a stamping on the engine to identify it also.