Put a tri-clapp cross at the top, then attach the blowoff to one side and the PRV to the other side. Get a tri-clamp cap, bore a hole through it and weld a long .5" dia. stainless tube to it. You can then put a thermocouple or RTD inside the tube and have it secured at the end of the tube with a .5" to .25 (whatever probe diameter it is) Swagelok fitting or by welding the sheath to the tube. Attach this long probe assembly through the top tri-clamp.
Do you have any fitings on the side of the tank?