I went with a Exide group 86 battery from Home Depot. This is a little shorter height and lighter weight than the original group 24 battery. Also, the stock battery clamp and plastic cover fit perfectly with this size. Battery output is also more than enough, at 550CCA.
As for tires... my '78 has 175/70-13 on the stock 13x4.5" wheels and they do fit and work well but if you're missing the factory wheel spacers, they will rub the inner frame rails or the lower spring perch on the front.
When I bought this '74, it had 155SR13 on the front, 165/80-13 on the left rear, and A78-13 on the right rear. Not the ideal choice. Now I am running 15x6 wheels with 175/55-15 fronts and 195/50-15 rears. It handles great and the wheel/tire weight is lighter than the stock 13" steelies+175/70-13 with this combo. Plus it looks awesome
Your '75 came factory equipped with 145SR13 tires. The factory wheels were 13 x 4.5". If you're out for authenticity, that should be the tire to run. The reason I run 175/70-13 is because that size has the most tire brand options in the USA and offers a little more tread. Also, with a 4 speed car, the 175/70-13 slows the speedometer down to make it more accurate. It also knocks about 100 rpm off your engine revs at freeway speeds and that's a plus.