The leak at the manifold is probably just a bad connection. For example I went through this a few weeks ago on my '72 MOPAR. Cut off the last inch of the heater hose to use better part and installed a better hose clamp, fixed that. Needs new hose really. The one at the firewall is more difficult. Believe it or not, the pressure in the system is higher after the engine is shutdown due to no coolant flow and heat from the engine continuing to transfer to the coolant thus expanding it. Be sure you have good hoses--resilient rubber, no pinholes or cracks or cuts. be sure the pipes from the core are round and smooth (no deep scratches or dents) and the solder joints are not fractured. hose clamps should be good quality and no bigger than necessary so they aren't too wide which spreads the clampng force over too mch area. Good luck.
My $0.02
