Re-fitting Hardwood floor - to Glue or not to glue?

Following on from the eventual fitting of UFH in our "upsidedown" house, we are hopefully going to re-lay the current floor. But there are a few questions to be answered to prevent mistakes.
Existing floor is 6m x 4m Lounge floored in hardwood strips about 2.5 Inches wide x about 3/4 Inch thick x about 2.5 foot long.
These "planks" go through the hall way to the front door. The rest of the floor (kitchen diner etc) is dodgy chipboard.
The planks are t&g but as they've been down since the 70's are pretty well seasoned. On re-fitting, should I just take up any gaps and nail every 2nd joist or so as currently fitted OR... glue t&g and fit as a solid unit like a floating floor? what sort of gap if required should be left for expansion/contraction?
TIA
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