I'll back up the advice to swap pan & pickup. It does fit with the X parts but the pan hangs down pretty far and the bottom is not level.
I suggest you use the 1500 long block and swap as many 128 ancillaries over as you can. The manifilds, dogbone engine mount & bracket, cooling system pipes and thermostat are a must. Flywheel and starter too. Other things like valve cover and distributor will work but the 128 bits are better suited. I haven't noticed exhaust clearance issues but all the 128s with 1500s I've dealt with had headers.