Upgrading to larger diameter wheels is a cinch if you adjust the overall diameter of the tyre to comply with the original.
For example I am running 205 / 50 x 15 tyres on my 1600 using Skyline alloy rims and the O/A diam. is within 3mm of the original 13" wheels and all is sweet.
The larger rim you use means the lower the tyre profile to get the same O/A diameter.
In my opinion there is a limit to tyre profile for these old cars.
Remember to consider clearance for the larger section tyres, you may have rubbing problems.