Here's an excerpt from an article by Martin Poole that was published in the Oct 1982 edition of  Enjoying MG, the monthly magazine of the MG Owners Club.  Any grammatical oddities are attributed to the difference between our "American English" and the "Queen's English".

The article explains how to flare the rear wheel wells, gently expanding them for larger wheel/tire combinations.

Thanks  go to Andy Wright for finding the article and providing me a copy of it and the diagram.

MGB WHEEL ARCH MODIFICATIONS
-Martin Poole

    "TO FLARE OR NOT TO FLARE or more correctly, if you don't you can forget about those beautiful 6J width alloys and the 185 radials that are intended for them.  The MGB is not the easiest of vehicles to work on with regard to wheel arch flaring - but it can be done very successfully provided care and restraint is exercised.

    In fact the finished job can be blended in and radiused so as to appear that MG intended it that way.  It is not a week-end job, actually the ideal time is during restoration of new wing panel fitment.

    My own V8 roadster rear arches were modified during restoration and were achieved in the following manner.

    Start by removing the chrome finisher strip along wing and by using an orbital grinder remove the paintwork in the arch section up to this line.  With the wheel removed and the car on blocks it can be seen that the inner and outer wing edges join together and lip under to form a finished edge.  This edge must be cut into for its full depth from the inside using a hacksaw with a cut every inch or so around the full radius of the arch, it then being possible with mole grips, a hammer and a wood block to flatten this edge flush into the inner wing.

Now taking ones hacksaw and courage in both hands make a point 6" up from the bottom of each side of the arch and make a saw cut right through the total inner and outer sections then proceed to repeat the process every 1-1/2" right around the arch.  Once this is done it enables one to gently hammer and bend segments made outwards increasing the gap between them.  Also it will be found that the inner wing can be flattened somewhat.  Provided care is taken and sufficient cuts are made the general contour follows on OK.

    Next, thoroughly clean all remaining traces of rust, paint and especially underseal from the area and polish with wire brushing for the brazing to be carried out.  We did our own brazing and found that it was easiest to braze the inner areas first working alternatively from each end.  Afterwards the joints between the outer facing wing segments can also be brazed preferably cooling with water between each joint to minimise distortion.  Once the whole side inside and out has been completed grind back the excess braze and remaining paint right back to the contour area.  It is quite in order to be very severe with the grinding and sanding as the grooving produced makes an ideal key for the next stage: body filler.

    At the mention of these two words 'body filler' a large number of members will no doubt shake their heads and say 'ah - body filler - botch job, lumps coming loose in the future'.  Suffice to say, as I have demonstrated to John Hill, Peter Laider and many others even after four years daily use the arches on my roadster are still strong enough for people to lift the car with and they have coped with a 200 BHP units stress without any cracking whatsoever...................."