I will second the motion. Heavyweights with Harriman style roofs are rare and usually expensive. The road I collect, SL-SF, had an extensive fleet of round roof cars of all types, as well.
Golden gate depot did a run of 60-C Harriman cars, but I agree they are quite rare. The 72-C cars would be nice to see in O and as many like the slightly shorter cars in 3 rail, they would fill a nice gap.
I have an extensive fleet of 80' GGD heavyweights in 2 and 3 rail. Probably close to 30 cars now. They can be found for fairly reasonable prices with some patience, especially on the coaches. The head end cars, diner, and observation are not as easy to find, and the 8-1-2 and 12-1 sleepers just slightly less challenging to find.
I had the opportunity this year to break in my E1s and a fictional ATSF mixed heavyweight / streamlined passenger train on a friend's layout in April. The streamlined coach and the diner are Lionel cars.