installing the SME V tonearm in the Technics SP10

Remember...the V has no adjustment for azimuth, so when mounting it on the table, make certain it is absolutely square.

Manitunc et al, I believe it is the square escutcheon that surrounds the platter and is part of the chassis that will get in the way, not the platter. The square also makes mounting of many tonearms with side pillars to adjust VTA difficult, awkward, or impossible. These include Triplanar, Talea, Reed, etc. That escutcheon is the best reason I know of to remove the motor from the chassis and mount it directly into a plinth.

Platter is not really over-size. It's pretty standard.

Stringreen makes a good point. Start with making sure the platter is level and the armboard is level. If both of these surfaces are level, or parallel to each other, then, assuming you can get the arm to fit, the headshell, which is what is critical here, should be also parallel or level to the platter and arm board.

Now, without azimuth adjustment, the cartridge build quality (cantilever orientation) becomes critical. If you have the optional SME V removable headshell, it can be slightly rotated to adjust for azimuth.

Lew is correct about the square escutcheon creating the problem, not the platter dimension. I also have a JMW 10.5 arm which would have been difficult to mount because of its off-set mounting base. Unless you swing the spindle to pivot distance template way around to near a 3:00 position for the arm, the square interferes with that mounting base.

Apparently Technics intended only 10 or 12" arms to be installed with this table. And since Tubeperson has the Obsidian plinth, he will be limited to those arms where the mounting template arc falls within the dedicated armboard position.

I have seen an SME V mounted on the obsidian plinth.
Here is an example of an SME IV ...
http://www.theanalogdept.com/k_mccarty1.htm
All he needs to do is make a template. Trace the SME mounting base, outline and hole, onto a piece of cardboard, cut the base out and measure the pivot to spindle of 215.35mm. Then just move the cardboard around on the arm board until it fits and the pivot to spindle is correct and mark up the board for cutting - pretty straightforward really.