Ahhhhhh... so this is a swap of a 940 engine into a 240...
Well, there are 5 3 ways you can get a speed signal without changing the diff.
If you have an early 240 mechanical speedo cluster (up to YM 1980, square speedo, with small tacho in the middle) you might be able to use a inline VSS device, which fits between the speedo and the speedo cable, possibly something like one of these - https://howellefi.com/product/tpi-tbi-vehicle-speed-sensor/
https://transmissioncenter.net/shop/54mg-eight-pulse-signal-generator-for-your-electronic-speedometer-head/
I haven't used one of these myself yet, but I'm planning to, since I'm fitting an LH2.4 engine into a K-Jet 240 and I don't want to change the speedo cluster.
.2. If you have a later model mechanical speedo (YM 1980 to 1984.5, carb or K-Jet, round speedo, round tacho or clock on the left of the speedo) you might be able to re-use the cruise control output of it to provide a VSS signal to the LH2.4 ECU. Details at the links below -
http://forums.turbobricks.com/showpost.php?p=5816970&postcount=13
http://forums.turbobricks.com/showpost.php?p=5943114&postcount=15
http://forums.turbobricks.com/showpost.php?p=5954896&postcount=20
.3. If you have a later model electronic speedo for LH 2.4 (YM 89 on, round speedo, round tacho or clock on the left of the speedo), you can replace the speedo drive at the gearbo with an electronic sender, and the VSS signal is then supplied to the LH 2.4 ECU in the usual manner.
Also, you may already know this, but there was a mixup with the labeling of the outputs of the Buchka wasted spark board - details at the link below.
http://forums.turbobricks.com/showpost.php?p=5711020&postcount=378