I've got a 1966 440 engine that I was considering planting into 1977 Plymouth Arrow.  Do any of readers know if this is doable?  

I think so.
Easy?
No.  

If you have the time, patience and money, it's all do-able.  After all, there were a few teams racing Arrows with hemis in them, decades ago.

You will have to find a better rear end, such as an 8 3/4, a Dana 60, or Ford 9".  Then shorten it and mount it.  You may need to alter the transmission tunnel to fit.  V-8 kits for the Arrow were sold in the 80's, but now you'll be on your own for figuring out engine placement and making mounts.  You should reinforce the front subframe and use elephant ears.  If you do get the drivetrain together, you'll twist the car apart, so a welded roll cage will help.  

This is a pretty heavy chunk of iron for such a little car, so don't expect any decent handling.  Aluminum heads, water pump, rear mounted battery etc. would help out though.