For V1/V2 units without the divider consisting of R342/R343, it is possible to make a simple modification to drop the reference voltage below 2.5v. As has been mentioned in other threads, the Vin at pin 3 of U16 is obtained from the junction of a pin on Q28 (possibly collector/drain) and R266 (100K). The other end of R266 has 2.5v from the voltage reference. Q28 is probably part of the voltage shutdown circuitry. It operates at cut-off in normal operation and does not enter this calculation.
The mod consists of a resistor soldered from pin 3 of U16 to ground, to drop Vin from 2.5v. Pin 4 of U16 is ground, so it is possible to solder a 0603 or 0402 resistor between pin 3 and pin 4 of U16 (10358). Since R266 is 100k, a 100k resistor between pin 3 and 4 drops Vin to 1.25v. By a curious coincidence, R202 is 10K and R203 is 7.5k on my V1, giving a Vcore of almost 2.2v (rated Vcore of K6-III) with a Vin of 1.25v on pin 3 of U16. To get other values for Vcore, R202/R203 may be adjusted suitably.
To summarize: Enable split voltages by moving R60/R61 to R184/R185. Solder a 100k resistor between pin 3 and 4 of U16 on a V1, to drop Vcore to 2.2v for the AMD K6-III. Solder a resistor in parallel to R302 to get a higher current limit if necessary, for clocks higher than 3 x 66 (default).