Best off using a late HX (6/76 on) or HZ chassis if you want RTS geometry. WB chassis is a bit different, but will work OK. You could 'fix' a WB chassis by using the noses off the HJ's rail, with a staggered joint using the fact the rails are really 2 x c-section halves. If one of those sections happened to have the HJ's chassis number on it, that can't be helped, just make sure you are up-front with anyone you may sell it to what was done. I've seen this done before to a WB chassis using a set of HQ sedan rails to fix it.