A 60" will probably fit. At least an inch or more of your tub edge on either end is behind the tile and lath surround. Once you demolish the surround, you will have the space needed. I just remodeled a 1940s bath and had the same concern. Both in length and width. A standard 60" tub fit my space. Good luck!

It also looks like you may have one of those plastic tub covers, meaning the original tub is bigger.

Thank you for your answer & reassurance! Actually, it's not a tub cover. We built this house 34 yrs ago & used poured marble which was hot stuff back then! I'd love to get rid of it, so I hope you're correct about the extra 2"!