From my calculation it should be 383. But apparently it's wrong.

You have done computations correctly, but the representation in base 8 is wrong.

45 in base 8 is 4\times 8+5 , thus 37 in base 10; 7 in base 8 is the same as in base 10, thus 45\times 7 in base 8 is equivalent to 37\times 7= 259 in base 10. Finally, you have 259=4\times 8^2+0\times 8+3\times 8^0 , thus in base 8 it is 45\times 7=403.

Notice that 383 in base 8 does not exists, as digits available are from 0 to 7; 8 in base 8 is 10, thus 383 becomes 403.