Not bad at all! That will be a cool car too. The big Ambassador was kind of classy if you ask me, especially if it has the continental spare tire and twin antennas on the back. I think the 60 has some small tail fins too.
You might want the replace the carb with a newer model Holley 600. The stock one is just a 450, and those things only got about 15-16 mpg. Lots of low end torque with the little carb, but I think gas mileage would even improve with something a bit bigger. At least it's a standard Holley bolt pattern!
Before you ask, there are no speed parts for that engine. It's a good reliable and smooth engine when running right, and should have plenty power for that Ambo. About the only thing you can do other than change the carb is get the stock cam reground to something a bit better, but you won't need that for a cruiser.