Classic/Antique Car Repair: Contemporary Engines in Old Cars, chevy 6 cyl, old chevy

Question
I have always loved the cars from the 1940's.
When I was growing up, our neighbor had a 1948 Chevy Fleet Master and I
have always wanted to own one.
However, friends that I know who own vintage cars, always seem to have
trouble with them or barely drive them on weekends or special occasions.
Also, as far as I know, older engines use a lot of gas, which is very expensive
these days.
Is it possible to put a contemporary, highly efficient, Japanese engine in an
car from the 1940's?
I'm sure this is sacrilegious to vintage car purists, but at the same time, it
seems like a dream come true.
Thanks,
Adam  

Answer
The problem is weight and the contemporary, highly efficient Japanese engine would probably get less milage than the engine that was originally in the car.

An old Chevy 6 cyl could muster 18-22 mpg and that is better than a lesser engine would get pushing that weight.

The amount of gas mileage is really very liitle when you compare conversion cost.