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As far as I know , they will not , your best bet is to call IMS or Clark as they make tanks for both (it would be hard if not impossible to get info directly from Honda , and I doubt a Dealer parts guy would know , so aftermarket would have more info) ..... As far as I know , the capacity is different , and more importantly , the frame spars on a 250 and 450 are different (R and/or X) , so because of that alone I would say no .....
The only suggestion I could offer is IF you can find a 250X tank , would be to match it up with your 450X tank , or better yet try to test fit , but I think the outcome would be the same , that the spar is different and it will either not fit between the spars because it is too small in width , or not bolt up as the shrouds would be different since the capacity is different ..... Like I mentioned IMS or Clark would know for sure as they have "fit " the tanks to the bikes and will know what the differences are and if they would interchange at all , even with mods ...... That would be your best option .....
Hopefully someone will chime in if they have tried it ,but I am just going by looking at the aftermarket tank manufacturers sites and viewing part # and capacities , and neither match , so that would indicate enough of a difference to make them non compatible , because they would not retool to make a separate tank/part # for each unless they actually had to due to differences enough that they could not make 1 part fit for both bikes , that would be my guess , as tooling for each bike cost them money , so if they are making a different part for a 250 and 450 , there has to be a practical reason and my guess would be physical differences in the bikes enough that it would not "bolt on" ......... Not a 100% direct answer , but , Hope that helped at least guide you in the right direction