I´m gonna put my money on no interferance. The biggest difference between the HO head and the LX head is the HO head has bigger valves. Your best bet would be to get some clay and lay it ontop of one of the pistons then bolt on the head and timing belt and turn the engine one revolution and see if there is any interfearance, then take the head back off ond measure the depth of the clay where the valves hit it to get your clearance. That is the way it is supossed to be done anyway. Also since you have flat top pistons you will have conciderabley less compression than the HO motor. If you really want to go all out you should get some HO pistons too, then you know the valves will clear, and you will have even more HP from the swap.