Mike,
2 types of 12G940 heads were sold out here. Note BOTH had 12G940 CAST on head under the rockers, but only some had this little stamped # near the thermostat hole:
12G938- Morris 1100S and 1300 auto, 33mm inlets, 29.4 exhausts.
12G1805- Cooper S Mk2, 35.6 inlets, 29.4 exhausts.
Also there was the 12G1316 `smog' head. Number is CAST on head under rockers. This head is basically a 12G940 casting with air injection ports in the front. Used on 1275LS and 1275 Moke. U can port it just like any 12G940. 33mm inlets, 29.4 exhausts.
Any of these heads can be modified to use up to 35.6 inlets with 31.0 exhausts without using offset exhaust guides.
<edit> but if fitting unleaded seats, 29.4 exhaust is about maximum.