727 Part Number

2538338 does come back to being from behind a 273 in 1966. I can't say what problems might arise from using the wrong switch as I have never swapped them. I haven't tried swapping rooster comb assemblies but I can't see why that won't work as I believe the valve body and manual valve is the same. One major problem you will have though is the converter hub on the earlier units were smaller than the newer ones so it will wobble around in the crank register and eventually break either the flexplate or trans. input shaft and possibly tear up the front pump too. There is a couple ways around it though. Some places sell a spacer that slips into the crank that makes up the diff. in size. You can also swap out the input shaft to a newer one and use a newer torque converter. Someone may have already done one or the other but you better check it out to make sure or as I said it'll tear things up.