Hi,
not specifically a SRX340 but I have a SRX300 at home put inside a drawer together with an EX2200-C-12P-2G. CPU temperature on the SRX is just around 76-77C and has been running like this for more than 6 months without any issues.
I have a friend who has a SRX340 chassis cluster where they are positioned correctly in regards to airflow and there the CPU temperature is just around 40C - so agreed your boxes are rather hot but I wouldn't be concerned before they go above 80C on the CPU (personal experience, not official Juniper recommendation).