You are pushing big pistons in big bores with stiff oil and low compression. Get yourself a Hawker Oddesy and forget the Chinese economy util you get your gummit rebate check in June. Then go to WallyWorld and support the Chinese trade deficit. Make sure all your connections are clean and free from corrosion and the terminals on the starter are tight, especially the top, booted one. Take the boot off and check it closely.
I have a feeling your Yuhasa's just fine. You need to load test it with a carbon pile tester. Check the drawdown amps. Just because a battery is slow to crank don't mean the battery is shot. Idle batteries will discharge over time.