One thing to consider with heritage breeds. They can fly. I had a mixed flock of BB bronze, spanish black and holland whites from welp hatchery. The BB's definitely finished out extremely similar to what you would buy in the store in approximately 16 weeks (I know you asked about heritage, just throwing in my experience). The Spanish ones were next in filling out extremely well. I think I harvested them around 20 weeks. I would not do the whites again, they were very slow to fill out. In fact when I harvested them with the Spanish they were not much bigger than a roasting chicken. I raised them all in the same place, same feed/water, same access to free range for bugs/greens, same housing.
Back to the flying, if you are okay with clipping wing feathers, which is a good option that I have no problems with, there is no good way to confine them. They literally roost wherever they want, even on the roof of houses/garages/outbuildings. I lost 2 due to fence entanglement and 1 to the neighbors dog when he went wandering.
That said, I will definitely be raising another batch next year. Probably a mix of bronze, spanish and narragansett.