how did you find out about the replacement 455 engine? did you check at the rear of the engine, where the tranmission meets the bell housing (12 o'clock position)? a 430 has big numbers saying 430 there: and a 455... well: 455
Going back to your original issue. The GM HEI distributor is commonly used as an upgrade to older cars. Should work fine. The resistor wire mentioned in a prior post is worth checking. If the hot driving issue is recent I might consider trying a new coil in the HEI distributor. They are known to fail from heat. Others might have a testing routine to use?
It doesn't matter if its a 430 or a 455 and you say it has the regular fuel pump on it that can be tested but I don't think fuel is your problem unless its vapor lock Buick HEI is almost bullet proof . BUT .... The Module and the pick-up coils will go bad and produce the symptoms you describe . Especially the pick-up coil . Check the wires for it and see they are not being contacted by the advance mechanism . It can be Ohmed to check . Usually 700-1,000 ohms when good but you would have to check it at moment of failure . It can get hot and " OPEN " and then when it cools go back to normal Modules tend to fail outright and never recover .
https://www.rockauto.com/en/catalog...455cid+v8,1021486,ignition,ignition+coil,7060 See what colors the coil wires are, should be red/white for Buick. https://www.rockauto.com/info/154/DR-32_Front__ra_p.jpg