http://www.ebay.com/itm/Cover-Access-H1 ... 27b896c6d2
This is what I bought. Haven't put it on yet, but I think it fits 3L80 and 4L80...
Also is the frequency converter you are looking for the one that mounts on the firewall somewhere and (I presume) halves the signal coming from the timing chain cover mounted engine speed sensor so it is compatible with the STE/ICE equipment (makes it output the same signal as the oil pump mounted engine speed sensor)? If it is, do you know if those work in reverse to double the output from the oil pump engine speed sensor?