re: extenstion cord
Yes, you can make an extention cord, and a properly made cord will work fine. (You mariners and campers can think of it as shore power.) There will be no difference in performance between a properly made ext. cord and a hard wired circuit. Bump up one wire gauge if you are going a long distance at maximum current, e.g.,> 100ft or so, for best results. 10Ga. is adequate for shorter runs at 30A; use #6 for 50A.
I use several large extension cords for heavy loads in my shop, both single and three phase, including one that carrys 90-->120A for a large arc welder, no problem. Oh, and before you flame me, I am a professional EE/ME, meaning I design these sorts of things for my livelyhood.