you can get clicking from siezed prop joint not noticeable with prop bolted up as hasnt developed free play yet or loose mudshield on diff or t/box flange
Really could be any number of things mate, if engine then: piston ring, a valve, a tappet, manifold gasket, camshaft, etc etc. Have ye got a helectronic device upon which you could record said ticking/clicking? Might help in our diagnosis.
sounds like drive train when under strain to me.Sounds like it will be a nightmare to find as car needs to be under strain for it to happen good luck old timer