Older Datsun/Niss sheet metal sucks. They are notorious for rusting. If i had one of these as a project, this is what i would do...
1. Strip entire car
2. Gut the interior
3. Repair and repaint body/chassis
4. toss the old engine and drivetrain
5. slap in a newer engine of choice. SR20DET's, RB's, 2J's, even a KA...Anything can fit in that bay without too many issues. A nice boosted SR will be super cheap and totally cake to throw in, not to mention reliable.