I would have been better off in a dress shirt with long sleeves.
この場合、次のような言い方ができます。
ーI should have worn a long sleeve dress shirt.
「長袖のYシャツを着れば良かった」
should have ... で「…すべきだった」
ーI would have been better off in a dress shirt with long sleeves.
「長袖のYシャツの方が良かったな」
would have been better off ... で「…の方が良かったな」
ご参考まで!