8x10 and 5x7 conversion backs are available, but they are not cheap. They each consist mainly of a tapered bellows and appropriate size rear assembly. Probably the best bet is to buy an 8x10 adapter kit and a 5x7 back to put on it.
Your 210 should cover 5x7 easily. Some of this length cover 8x10 when stopped down, many do not. Unless the 135 is an extreme wide angle it will not cover either of the additional formats.