If partner's 3nt is based on a club fit such as Kxx or even Kx and scattered values in other suits, 3nt may easily be the best spot. If you need time to get clubs going however, 4♥ or 5♣ might be safer.
If you did try 4♣ how should partner treat it ? Should he co-operate by bidding his major suit naturally if he was dealt one or would he be reluctant to do so for fear that it might be interpreted as a cue-bid in support of clubs. In other words, would he interpret 4♣ as exploring an alternate strain rather than slam ?

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(2d)-3c-(p)-3nt-(p)-?