Why is there excess heat from the manual transmission tunnel 1985 Nissan 720?

have you checked the fluid level recently? if the oil breaks down it tends to overheat. there is a check bolt on the side of the tranny, if you cant feel the oil with your little finger its low... would be a good idea to replace it anyway. depending upon your preference, driving conditions and climate 80w or manual trans oil. dont use atf... best to replace it about every 100-150k miles anyway