So if i understood right you want to fill the Tiles the entire Layout Size but your problem is that that you cant fill out the whole layout cause they would be just half on the layout. So if thats your problem you must change the Layout size to something that you can divide with 70 like 700*1400 or 1400*700. You could change the window size like this too. The secound possibility is that you open the tilemap in for example inkscape and set the export size to Tiles in the widht * 64 and the Tiles in the height * 64. For example the widht to 4*64 and the height to 10*64.
I hope i helped you.