Substituting milk with whipping cream
As the previous answer says, there is an enormous difference between milk and cream.  

However, it is possible to reach a rough substitute for milk by mixing your cream with an equal amount of water by volume - in your case, 3/4 cream and 3/4 cup water.  

It won't be exactly the same (the fat content will still be substantially higher than it would be if you used milk, and if you do a taste test, comparing the flavor of whole milk to a 50/50 cream/water mix, you'll see that the flavor is also very different), but it should work.  

However, there is a chance that the higher fat content will lead to the sauce breaking, so if you want to be 100% sure of success, go get some milk.  If you're willing to gamble a bit, try diluting the cream.