As well as having to change the fork slider to accomodate the brake pads for the disk brake, if you have a stock 1970 front fender you will have to ensure that the tyre you put on is narrow enough to fit within the fender stays - the stays and tyre on the 1970 and earlier are narrower than on later model bikes.
I put on a slightly wider than stock tyre on my 1970; and had to have inserts welded into the stays to widen them, and lever the fender out slightly so that the tyre would fit in. I am not sure if the rim itself got wider after 1970 - and if it did it probably doesn't matter provided that the rim will take a stock 1970 tyre; so if you do want to stay with a 1970 type tyre it might pay to check the manual (I think that there is one online on this site) to find the correct size tyre, and double check that a stock 1970 tyre fits this rim - logically it should, but I have found that some of my seemingly logical assumptions have been way out.