I have to say I like the arguments that if you're going to bid a slam, you should bid it in diamonds. There may be some people in 4
♠ (assuming the wheels haven't fallen off) and you will score well finding any slam otherwise.
There are a couple of points of interest. First, I was under the same impression as most of you that partner was showing a 65 hand. If you don't feel that, then explain how partner is able to show a 65 hand.
Second, partner's 3
♠ could be interpreted as either (a) a natural suit, (

a stopper looking for no trump, or © an advanced cue-bid (when he bids on over 3NT). When he bids 4
♠, I think we can rule out (

.
Third, partner thought that when I bid blackwood, DIAMONDS were agreed. I felt that SPADES were agreed. (This was only the second time we played together to be fair.) So when partner showed 2 keycards with the Q of trumps, I know we're off 2 keycards. So unless I'm banking on a void in partner's hand, I didn't dare go past the last making contract. That didn't seem to work too well! Although interestingly enough, 4
♠ makes if you let them win the first 3 club tricks discarding hearts in the north hand.
1♦ - (2♣) - 3♣ - (P)
3♠ - (P) - 4♦ - (P)
4♠ - (P) - 4NT - (P)
5♠ - (P) - ?