Cannot install Leopard (Mac OS X 10.5) on PowerPC Mac

I have an old Power Mac running Tiger (Mac OS X 10.4.11). It is a Dual 2.3 GHz Power-PC G5 with 8 GB RAM. I have it running Tiger since that is the last OS that supports Classic (OS 9) (via Rosetta) and the kids had many Classic programs and games that they like. As far as I know, Leopard (Mac OS X 10.5) is the last OS that can be run on this machine. As it has 2 internal HD slots, I installed a 2 TB SSD in the empty one in order to install Leopard on it. The SSD is partitioned into 2 discs, 1 TB each, and formatted according to the Apple Partition Map (not GUID). When trying to install Leopard OS from an original DVD, and choosing one of the SSD drives, the installation starts and stalls after a few seconds. The whole machine freezes, and I have to do a hard reset. I also have an original Mac OS X 10.5.6 disc (© Apple 2009) and when I try to install from it, I get a window saying "This Operating System cannot be installed in this Mac" (!!!). I then created a bootable USB flash memory with the iso image of Mac OS X 10.5.6 downloaded from the Internet. My Mac does not see it, even if I boot while pressing the Option key. So I have not been able to install Leopard on a machine which is supposed to be able to run Leopard. Any help will be appreciated.