There are a few body changes that were made in 1969 that moved the axle line to the rear some inch or two. This was done to make the 911 handle better. It shifted the CG forward and gave the car a more neutral polar moment. The car had less of a tail hanging out effect in turns. The 912 (4 cylinder) engine is much lighter than the 911 (6 cylinder) and by transplanting the 6 cylinder engine from a '78 thru '84 911 will make the car handle worse than you are currently experiencing. It is more than just the engine that would need to be changed if you went forward with this project. The current Piwis Tester II Type 901 transmission is not very strong and you should change it with the matching engine. There will be the addition of the other support systems like the oil tank and valves associated with the dry sump oil system; addition of a high fuel pressure pump and return line; addition of the engine control unit computer (ECU). I would also consider better more effective brakes with all the extra horsepower you will see. The suspension would need some attention as well. The 912's suspension is not really made to handle the larger, heavier, more powerful engine that you are considering.

Now you need to consider the cost involved with such a swap. A 911 engine is not cheap to rebuild. Which translates into good engines are expensive to buy. I spent about $4,000USD in parts to rebuild my '72 911T 2.4L engine in 1987 and did all the work myself, short the VCM II cam regrind, and it was not a race engine. I drove it for over 150,000 miles without a problem. So a good engine and all the accessories are going to cost almost as much as a good complete car. I would sell the car you have and buy the newest car in the best condition you can afford and not try to swap the 911 engine to your 912.