The grille and bonnet - errr...hood are the main external distinguishing feature and you have them. The rest is pretty much trim level. Yes, you could use the '72 parts to fill in the blanks, albeit with slight modifications. The model would deserve to be built fairly stock IMO, just to show off that it is a rare 68 annual and not a common 72.
The six years of production, from 1967 to 1972, can be divided up into three groups of two years. The '67-'68, the '69-'70s and the '71-72s.
1967 and 1968 they shared the body introduced in 1967. The only real difference between the two is that the '68s had side marker lights. The '67 pickups were the only ones of the bunch which could be had with a small back window.
1969 and 1970s have a more upright leading edge of the bonnett - errr....hood and thus different grilles. The bonnetts and wings - errr...fenders are interchangeable on 1969 through 1972 trucks, apart from the mounting holes for the bowtie emblem on the '69 and '70 hoods. The badge was moved to the grille on 71-72 models.
1971 all Chevy light trucks got front disc brakes and hence different wheels for the half-tonners (and suburbans). The 1971 and 1972 trucks have yet another different grille without the center bar used '69 and '70.