Turning it into a cruiser would require a heck of a lot more than a change of tires...
You can easily turn into a motard-style bike by fitting it with sticky street tires, better brakes, and a lower from fender.
You can sort of give it a cruiser(ish) look by fitting street tires, high-rise bars, a sissy bar, and some low-level exhaust pipes (pipes made for a CB175 or CD175 should fit). To complete the look you could shave the front part of the seat to make it look like a stepped saddle, or simply swap it out for a single tractor-style sprung seat. Then you need some type of peanut-style tank. But, the single biggest mod consists in raking out the front-end. The simplest and safest way to achieve this is by fitting longer fork sliders or a complete new and longer fork assembly.
My 2¢ worth, though, is that what you have is a very nice little bike which is now becoming quite rare and so might be worth preserving in stock condition. Why not leave it alone and instead just spend the money on an early mid-size cruiser like a Yamaha XS400 Special, or a Suzuki GS400L, or Honda's own CB450SC Nighthawk. Decent examples of all these bikes can still be found for very little money. You'd have yourself a cruiser and your SL would remain unscathed... That's just my 2¢; it's your bike.