i have a beautiful 1975 Nishiki Custom Sport which was given to me by its original owner and am in the process of replacing some of the components. i am tempted to go with shimano 600 for everything, keep the japanese theme alive. i would like to forgo a front derailleur and only use the larger chainring, i usually only use this when riding. i feel that this would clean up the look a bit. would this create any problems if i kept a 5-speed freewheel on the back?

well, 5 speed freewheel ≠ Single speed so this is kinda miscategorized , but yes it will be fine

Unless you live in a flat area or are just commuting then I'd keep both chainrings. You're still riding multispeed so why not keep it as a double crank? I don't think the looks will be affected that much

Yeah, aesthetically I've always found taking the Front derailer and the shifter off looked kinda ugly.

I have my utility bike set up this way, though, mine has an 8 speed in the back and 1 in the front Its like the 1x9 mountain bikes. I know I'm happy with it.