OK, the 1977 and 1979 models are completely different.  The 79 uses the nice integral unit that fits between the stern brackets.  The 77 model uses the external bolt on unit.  So, the 79 system will not fit the 77.

The 72 100hp tilt system will not fit on the 1977 70hp model.

There were two systems available for the 77 70hp:  an external power tilt unit that had no trim capabilities, this system swapped out one of the internal shocks for a hydraulic cylinder.   There was a power trim/tilt accessory unit available for the 77 70hp as well.  This system is easily identified by the two external hydraulic cylinders mounted outside the stern brackets.  This kit came with hardware that disabled the original tilt lock used with the manual tilt models.

It will be tough to find a used power trim/tilt kit for this engine, and it won’t be cheap when you do.  Avoid anything that was used in salt water.  Is your 77 70hp a salt water engine?  If so, it would be quite the project trying to get it apart to replace the original gas filled tilt assist shocks.  This is not a bad job if your engine has only been used in fresh water.  So, I guess if it were mine, I would look for decent used/new gas filled assist shocks and replace them..

You mention using the 79 trim/tilt unit on the 77 70hp.  Is there something wrong with the 79 model?  Bad powerhead??  If the powerhead is shot on the 79, I would just install the 77 powerhead on the 79 midsection to take advantage of its power trim/tilt.  Yeah, the powerhead is not identical to the 79, but it will fit perfectly and work just fine.