good luck getting a boss 429 into a '68 without some major mods- it took a shoehorn for me to get a little 351C into a '68 coupe (and it took Kar Kraft a bit of work to get them into the roomier '69-'70 models)
Actually they did come with a big block 390 option back in 68, friend of mine had one. Changing some of the plugs was near impossible but the motor did fit. Not sure about the difference in size though, 429 hemi or 427 cammer are probably wider
