If the LU won't separate you missed a bolt under the trim tab, hex socket, viewable from under the anticav plate after removing the trim tab. The 4 cyl 70's had one there. Haven't been into the smaller LU that the 3 cyls use so you may just have the 4 bolts/nuts on the side of the mid section..
The shift shaft in the LU can come out and is a mess to get back in. As you separate, pay particular attention to that shaft (male spline on the LU and female on the mid section which is right in front of the LU. Ensure that the top comes up and the bottom stays put....whatever it takes.
If you move (twisting action is what shifts) the shift shaft or the engine shifter with the LU dropped you have to realign the splines so that the shift positions will correspond....what you ask for and what the engine gives you before you couple the shift shaft back together. Come back on here and ask how to do it.
I don't know if you have a bracket to support the water tube (from the water pump outlet on top to the engine block) or not, close to the LU. If not you have to stab a hole in the bottom of the engine block so make sure you get that done. If you have any seals on the tube or top of the WP ensure you get them back where they were.
The impeller is held against the driveshaft with a key that slides in place and can fall out when you lift the impeller out of position. Watch for it and ensure it gets back in...or the impeller won't follow the drive shaft.
Inside of the WP housing will be concentric wear rings but none as deep as to catch your finger nail. If so, new pumpl
When pushing the cover down over the impeller blades, lube them up good with soapy water and with the impeller and key installed, push the top down on the impeller and as you do, turn the crankshaft CW looking down on the LU.
If the LU won't reseat up against the mid section, the last ?-1" the drive shaft splines aren't aligned with the splines of the crankshaft so one or the other has to be turned till mating occurs.
Edit. If your impeller blades have pieces missing, find them. Can/will find a way to hurt you down the road by plugging a water passage somewhere.
Good luck.
