1978/80 oughta share mostly compatible parts - biggest difference around that time was EGR valve and blowby oil/vapor separator occurring on later models starting around 1980/81. Heck y'all 1980-85 n/a 617 owners might want manifolds from the '78 as "retro-upgrade" ditching the EGR entirely. Meanwhile different glow plug system though, bein loopers instead of pencil tips. And it shares the non-bowden cable tranny with yer '80 model. Also check the rims for being highly desirable black painted alloy hubcap style assuming they aint bundt-cakes. Early '70s 113/114/115 owners want those alloy wheels to upgrade earlier steel hubcap rims. Also count the upholstery ribs on front seats - should be 5 instead of 6 that occurred on later models. This means if you swap interiors, take both drivers' and passenger seats to keep em uniform. Meanwhile all kinds of fiddly bits like door straps, catch plates, central lock diaphragms, window motors and regulators will make it worthwhile. And yer 240D pal will want the larger 300D fuel tank.