A K Hamilton Jenkin's 'Mines and Miners of Cornwall' Vol XIV (St Austell to Saltash) has much of the information you need. The adit to which you refer is identified by AKHJ as Wheal Tamar, driven 1000' west from just above Saltmill Creek to terminate under Longstone.
Having grown up and gone to school in Saltash, and living about 10 minutes' walk from the adit entrance, I have clear memories of seeing its gated-off entrance on one side of the lane that led to the Creek -where there was an extensive Council Tip that was slowly being used to infill the Creek itself. I also recall the copious amounts of water draining from the adit.
If Wheal Tamar did indeed reach Longstone, it must have been virtually beneath our house. It certainly must have penetrated the pillow (or submarine) lavas that create the high ground here and on the Saltash High School new site up at Wearde.
I encourage you to access AKHJ's book.