Will the lower units on Mercury outboards inter change?

I have a 1973 1400 that has no reverse will the lower unit off of a 1978 1150 interchange

Also have gas running out of the carbs in the front are the floats stuck?Will the lower units on Mercury outboards inter change?Go to mercury outboard shop they can look it up fo sure%26gt;

Needle seats need replacing%26gt;Will the lower units on Mercury outboards inter change?I have heard that they will bolt up, but the gear ratios may be different.