If the radiator is steam then you have slip nipples which you nee to diconnect and get some good wide wood wedges and lay rad. down flat and put wedge between fins. by each nipple and hammer them even until they spread apart. Has to be even are they wedge. If it is hot water dothe same but where valve is you need to take out. First thing you have to buy a radiator tool that fits inside the nipples and a 24″ pipe wrench and maybe a cheater. and lots of muscle. You might think of getting a price to do this work because the nipples inside the radiator have right hand thread on one end and left hand thread on other end, if you cross them you will have a leak.
Art retired plbg