First off, the #8al is not good for a 50A circuit. What is the kW rating of the new cooktop. With #12 leads I would think it is around 4.5 - 5 kW.
Be sure to replace the breaker with the appropriate one.
I would use NSI IT-4 Insul-Taps for the conncetions. Super simple. Super safe. Super easy. The only thing is they are not super cheap.