3G engines will bolt in 4G cars. 4G engines won't bolt in 3G cars, because of differences in tranny and engine mount design. Bolting the 4G engine 
and tranny in a 3G is straight forward and will work. 
So to answer your questions, the good ole 8 valver will bolt straight in using all the 4G sensors and stuff and ghetto fabbing the 8V dizzy to work w/ the 12V dizzy setup. 
You won't have to do that if you bolt the 4G 12V head on the block. I just don't know, what head gasket to get.  

 People have converted 8V engines to 12V by just bolting on the 12V head.