Yes, it has to be a '68-'71 booster, the '72 and later boosters are too big around to fit the available space. As noted above, you would also want to use the 68-71 master cylinder as well. The 68-71 booster uses the same mount flange so it should bolt straight in. If the vehicle does not have a split brake system already as part of the previous conversion, you will need to upgrade to that configuration to use the 68-71 master cylinder. That will involve either installing a brake portioning valve or a 68-71 metering valve. Note that the 68-71 brake system used a metering valve installed at the front cross member split, it did not use a portioning valve. The distribution block below the master cylinder on a 68-71 brake system is a brake warning valve, not a portioning valve. You can safely install a portioning valve below the master if you choose to do so. Some applications will require an adjustable portioning valve to rebalance the brake system. Note that all tubing to the front disc brakes should be 3/16" if that size is not already installed, it will have to be upgraded to the larger size.
Dave