Try the new antenna before spending money on an amplifier. 325 yards should be easy for that transmitter, even with the stock antenna. Mine, which is a mile down the road, is actually about 820yd away via straight path, but that path is heavily wooded and it still works good in the house. I use a baofeng for the receiver though. I am surprised that 325yd is giving you an issue. Is your receiver in a building with metal siding???I have 325 yards from transmitter (MURS MAPS) to receiver (M538-handheld). Tall grass, dirt mounds, etc, may be what is giving it trouble. I just bought this antenna which hopefully will help on the transmitting end. I have been having a bucket-load of unreliable-communication issues up to this point.
The amp will be powered by a huge 12v power-supply, not by the batteries in the MURS MAP.