I've replaced several hoses w/o removing the manifold but of late I've gotten into the habit of pulling the manifold to inspect the gasket. I've had several cars with intermittent idle & MPG issues and sometimes it's been a weak manifold.

But it just takes patience if you don't want to do the manifld.

Good luck on the overheating. IME 240s won't take much and to date I've not had overheating not warp the head.

I did mine a few years back (100K miles give or take) any went ahead and just removed the intake to clean everything under it up including replacing the oil box that's under there in the bowels of it all and an chance to replace the gasket. Good luck.
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'86 sedan with 447,000 miles on the original engine and automatic transmission

I've never taken off a manifold to replace heater hoses on a 240. The trick is to have tools to reach the hoses/clamps. I use a 1/4 inch drive ratchet, long extension and a 7mm universal socket to undo and tighten the clamps. Carefully slicing the hoses makes them easier to pry off. Long needle nose pliers help to guide the new hoses into place and a bit of soap or spray silicone helps them slip onto the outlets. If there's a running problem related to vacuum leaks at the manifold it's a different story. If the hood keeps you from getting a good look and angle at the connections there are only four bolts, a lamp wire and a ww tube to deal with to remove it (assistant helpful). -- Dave