Should I buy a 1970 Ford F-350?

"It has about 180k miles on the dash "

Highly unlikely, the odometer only reads up to 99,999 miles, then it turns over. Six digit odometers were still about two decades away in 1970.  Unless the owner has a stack of receipts regarding the rebuild, assume that it didn't happen.  

If it’s the one in the photo no way. That’s just a heap , one step from scrapping.

No, go get yourself something that is worth your money.

Unless you really know how to do mechanical repair work on one I wouldn’t, at an age like that you will be seen once a week underneath it with your arms up in it. 

That "big motor" will get you about 5 miles per gallon of premium grade gasoline. NOBODY needs a 1-ton pickup truck for basic transportation.

Just be prepared to get about 9 MPG on the highway and less around town.

That depends on what you want to do with it. The mileage is completely irrelevant, engine or otherwise. If you want help then go here  http://fordification.com/ for just about anything. You can even hook up with other enthusiasts in your area

As a hobby truck and something to do around the house, yes.  As a daily driver, no.

I would have a mechanic check it out thoroughly first, just like with any used car.  The last thing you want is to buy a truck that old and the engine or transmission fails.  If he says it's OK, go ahead and get it, if you like it.

One more thing.  No one can tell you if it's a "good buy" if you don't give the price (and mileage).

i dont see why not