Furnace and Water Heater Exhaust

With 3 daughters and a wife all taking showers/baths now our poor little 40 gallon water heater is just not cutting it. So I just purchased a 74 gallon water heater to replace it.

Current Setup: 120,000 BTU gas furnace that has a 4" exhaust pipe. There is about 2 feet of horizontal run before it expands to a 5" pipe. This is where the current water heater exhaust joins in. It then has the 5" pipe that runs another 5 feet of horizontal run before entering into the brick chimney which then goes about 24 feet up to the top of the house.

The current water heater (40,000 BTU) has a 3" exhaust that joins in with the furnace exhaust where it jumps to 5". The new water heater is 74,000 BTU and requires a 4" exhaust pipe. The 5" that runs to the chimney doesn't seem like enough exhaust throughput to support both the furnace and new water heater.

Will I still be ok with this? Does the 5" exhaust need to be changed to a 6" exhaust? Where the 5" goes into the chimney does the tubing continue to the roof typically or does it end and the exhaust goes up the standard chimney liner?

Thanks in advance.

Dan