Trouble codes

[leftmullet4]

I have a 87 chev 4x4 w/350 tbi.
I swapped computers but forgot to swap prom chips. I have been getting a code: 24 vehicle speed sensor, and code: 42 elec. spark timing control.
Does anyone have any expierience with these codes??
Yes, i have checked the wiring about three times and i can't find any shorts. I think that because the prom is different the computer isn't reading the information like it should, or, the spark table is set up differently. BTW- the computer is out of a 91 burban which should've had the same motor.
any ideas, comments??
thanx -andy