I think we should collect more information on which cars benefit the most from which engines. I mean in the sense of comparing the maximum power of a fully-tuned car with its stock engine with that same fully-tuned car with an engine swap. That way, when an engine can be put into multiple models, we can better-determine which car is best.
For example, I took a look at GTS' stats on the Pantera and Mustang Mach 1, and then I compared it to those of the Mustang GT '15, as the last of which has the engine that can be put into the first two cars I mentioned - the Coyote 5.0L. It would seem that if the maximum power of a car in GTS is the same as it is in GT7 - including special parts - then there's not too much difference in power between say, a fully-tuned Pantera engine and a fully-tuned Pantera with the Coyote 5.0L.
Now, it's not like I'm going to actually spec out a swapped car to have that maximum power all the time, as it may simply become uncontrollable. But it'd be nice to have a broader range of options, in case there's a regulation of HP or PP where the car can only reach that metric with a swapped engine.
The two major examples of this would be the Lancia Delta and the Mini Cooper '65, where they are ideal candidates for the GT-R Nismo engine and the K20, respectively. That's what I want to hear more about. Similarly, I recall the 2JZ-GTE can be really nice on the GR Yaris. Not only will being a 4WD car likely help handle high power outputs, but the GR Yaris has less power fully-tuned with its stock engine when compared to say, a fully-tuned GR 86. So putting the 2JZ in the GR Yaris seems to be a splendid fit since it offers it a broader range of HP depending on your power restrictor/ECU settings; it's even a swap that's actually occurred IRL, too.
I think it's also a bit early to determine this when an engine can only go in one car, or if more than one car seems good. Respective examples would be the engine that can only go into the DeLorean S2 so far, and the Porsche engine that can be put into the two older Volkswagens. (Though I'm totally putting it into the Sambabus for giggles.) For that Delorean, that Rampage engine does seem to boost its power a lot, but I want to wait and see what else we might be able to put it into.