...5/8 go the 5/8 ,or the 3/4? 3/4 tube on htr.core is on top,,5/8 is below that,,3/4 on the engine comes off of the t-stat housing,5/8 goes back to the water pump. 91 YJ 4.0 engine
pretty much look at it really and really one wil not type the other so if one slides on and the other done switch hoses and size is on hose cant see any problems sounds like you have your answer already