Imo, a Cavalcade wouldn't be that bad, as if we looks into NFS:U2, there's a Cadillac Escalade that looks as the same.
Even if we need to re-model it to make it feet more NFSSA, it wouldn't be a shame to get one in the game at all. But it's only my opinion.
Btw for Boxville, it's shaped as the real delivers cars so it's normal that it got that "old" GTA style, it doesn't need reworks to fit the games, look at UPS delivers, they haven't changed in a while, so why would they change in GTA:SA ? They just needed to get it "polished" and then it's done, it fit the game and 92 perfectly.
Even if we need to re-model it to make it feet more NFSSA, it wouldn't be a shame to get one in the game at all. But it's only my opinion.
Btw for Boxville, it's shaped as the real delivers cars so it's normal that it got that "old" GTA style, it doesn't need reworks to fit the games, look at UPS delivers, they haven't changed in a while, so why would they change in GTA:SA ? They just needed to get it "polished" and then it's done, it fit the game and 92 perfectly.