I hoped moisture would not be a problem, but if it is high like you state how would that affect the outcome, Is it still warping potential?
Two possibilities, or both: warping (most probably cupping) and/or the joints might also start to pull apart. It might be worth getting hold of a cheap moisture meter and checking all the surfaces just after they've been cut. If you get big difference between surfaces/points on the resawn boards then it will all need drying out some more until it stabilises. BTW the thinner it is, the faster it will dry
And glue and biscuit was my plan although i hadnt thought of clamp problems just yet? Planing and the centre cut will take them down a little but as you say will still be heavy, one is 2.8m long!
i dont suppose screwing them underside with braces in situ would be goer?
Not really. The only way to get an acceptably tight joint is to cramp it all together. Screws will achieve SFA IMHO. And make sure that you have a sharp jack plane and a belt sander to deal with the inevitable joint issues
I'd sincerely recommend (again) going down to 35 to 40mm for the top thickness; thinner material will dry quicker if there is an MC gradient surface to core, it will be a lot lighter (have you checked if your cabs will stand the weight?) and will be easier to man-handle (can you lift it?). Your hob clips, etc will also work with it (I doubt they'll work on 65mm stuff). Oak has an approximate density of 590~930kg/m3 (with a median value of 750kg/m3) so your 2800mm worktop at 615mm wide x 65mm thick (0.112 m3)
would weigh between 66kg and around 105kg (84kg avg) - a 40mm top would weigh 41 to 64kg (52kg avg). I've installed my fair share of oak counters in pub/restaurants and if they are needed at 60 to 90mm deep they come with planted-on skirts, but the main part of the top is generally around 35 to 30mm thick - partly the weight, partly stability, partly the problems of handling the heavier pieces. The thought of humping 84kg chunks of oak about without damaging anything, or myself, makes me blanch somewhat
I agree with B-A that narrow staves are more stable, but they're also more work. IMHO they don't look as nice as wide stave, but that's just personal choice. If you go that way you really will need to make sure that all your staves all have the same MC. You'll also find it mighty difficult/time consuming to deal with the inevitable joint issues you'll get, especially if any time elapses between planing and glue-up. It's worth noting that a worktop manufacturer deals with these by running everything through a large wide belt sander several times - I mean one of these:
(about 8ft high and £25k and upwards)