The URM-3-313 is listed for a 50-51 car automatic (235 Powerglide). I think it would have the wider base.
Hope that the URM-3-312 has the narrower base (assuming you have the narrower base intake manifold). The URM-3-312 is listed as working on a 216 with manual transmission - the narrower base manifold.
Good luck - please let us know how this works out.
