The Willys passenger compartments were small in the first place but with the amount that has been taken from yours already I would find it very difficult to drive with any reasonable expentancy for comfort. Believe me when I say from experience that trying to put seats big enough to be easy on the tush and still have leg room, even for short guys like me, is a challenge and then with the size of your firewall box and tunnel shape, you will find it hard to have both a brake and throttle without the tendency to hit one or the other when you don't want to. I know you want to keep it as correct as your father-in-law had it but perhaps he wasn't far enough along to really understand the drawback as well.
With both the sentimental value and the market value that your car will have you owe to yourself, and to the next owner, (let's be realistic somebody else will own it even it it is your kids), to make it as driveable and thereby as valuable as it can be.
John