swaping 19" front wheel for an 18"?

Most of the 70's racebikes did this.Adding not only a smaller diameter front wheel for quicker turns, but wider rims also.Im wondering If this can be achieved on a 70's streetbike,and would you have to lengthin the forks to make up for the lower front end.You dont want to put too much engine wieght on your front tire right? Has anyone done this or would it not work for a streetbike setup? Any help would be great.