There are several things which could effect no start when hot. First of which is the posiblity of heat expansion due to the non stock manifold/ exhaust. You may try to rig a temporary heat shield to see if that improves the condition. Failing in that I would suspect the starter itself. You say it is a rebuilt from a chain store.They are not the best place to buy a rebuild from. I have have some failures when I went that route. There may be some malalingment or warpature in the shaft in the armature which when it heats up causes it to bind. There is also the possiblity that the rebuilder did not do a proper job cutting the comutator.Which you would not know with out disassembling the starter. I assume you did not have this problem with the old starter before it gave up. Since it is life time I would next proced to getting a different starter. Good luck. keep the group informed on your progress George 1964 LeBaron Manteca, Ca