rockers will help, but realistically you would need a ported blower and LIM, and ported heads to run just a PLOG and downpipe.
The LIM will work with either engine, but I would go ahead and open up the inlet big enough for a Gen 5.
I got my Gen 5 for 250, most gen 3s sell for 150-200, depending on condition and if they're modified/ported or rebuilt.
A 3.8 won't give you that much in kr problems, but considering that a top swap is considered a "high level" modification for an N/A, it's always better safe than sorry.
