Long and narrow will have side to side stability issues, but shouldn't be too bad. I like it!!!
Check to make sure the foam isn't wet before putting any work into it. You can take a core sample by shoving a 1/2" pipe into the foam and pulling out a plug. Check it at the bottom, if the plug comes out dry through and through you're good to go... if it's wet it will need to be replaced. Wet foam can add more than 100 lbs of weight (sometimes more than that) and will slow the boat down and cause even more stability issues. Wet foam also won't float your boat in case of an emergency, it'll go down to the bottom.