Large excursions in thermocouple readings with MAX6675
Chances are that this is a thermocouple issue. I doubt that this is an SPI bus issue.

MAX6657 is intended for a a floating thermocouple.  When you connect the tip of the thermocouple to the aluminium block, it's no longer floating.

We have a thread about floating vs grounded thermocouple.  See this answer and the discussion in the comments under it.