ls1 is a chevrolet engine, not sure if it's a camaro v8 or a vette v8, and it's all american muscle, means it's na but u could get it from chevrolet, but to make life easier, there is a company that i deal with called auto atlanta, they have all the possible conversions that are common practice, from turbo's to the ls1. regarding the rb26dett, i know it's a skyline engine, and i know a lot about skylines, i doubt in any way that u can fit a straight 6 into a 944 engine bay, then to ad the turbos, i doubt it's phyically possible. but if ur willing to spend the money on importing the engine, getting the car(944) to meet emmisions, then y not modify the chassis of the 944, and put a caymen engine in it. i also posses a great book, in it they actually mention that when porsche first came out with the 928 and 924, they built a 928 clone that could b setup in all 3 version, rear, mid, and front engine.
to answer the ? rb26dett, good idea, but not worth it, ls1, u'll get 350hp/350tq, so yea, better get a 968, 3.0 4cyl, 300hp