My son has a 1994 Chevrolet 4x4 pickup with a stock 4.3 engine automatic transmission. When his engine went out he replaced it with a 350 V8. Soon as he fired it up the transmission started acting up. Shifting bad, lugging skipping gears ect. My question is since changing from a V6 to a V8 does the computer need to be changed or reprogrammed or is it that the old transmission just can't hold up to the new V8? The transmission was doing fine before changing the engine.  

How has the engine been running? Generally, when you put a bigger motor into a vehicle that came stock with a smaller one, you need to either have the computer re-flashed, or you need to get a new one. Your computer still thinks it is running a 4.3L so the amount of air and fuel your engine is getting is not going to be the right mixture/amount. Not sure why your tranny is acting up, however. I would recommend getting the computer reflashed by a reputable custom tuning shop. This will improve the performance and overall health of the motor. It may also fix your tranny problems. To find a good tuner, search google for "custom pcm tunes".  

While a SBC V8 will bolt to a SBC derived V6's transmission and both are identified as 700R4/4L60 or 4L60E, they are not the same transmission at a part for part level. Those used behind the V6 are not as robustly built as those behind a V8. This goes further in that those used in S10/15 with the 90 degree V6 are also not as robust as those used with a 90 degree V6 in the full size P/U and on and on this "Design For Cost" thing goes.

The robustness issue is compounded by aging. An automatic wears out at about the same rate as an engine. That's to say as an engine ages its power gradually drops off till overhaul or replacement becomes necessary. An automatic transmission attached to same engine looses its ability to transfer torque/power as it ages at about the same rate as the engine looses its torque/power to age. So they both end their useful lives at the same point.

If you did nothing more than attach a rebuilt V6 to the existing transmission it probably wouldn't have lived more than a month as it couldn't react the higher torque/power loads imposed on it by the fresh engine. Attaching a 350 to the old transmission just made things worse by at least another 33 percent.

And yes the computer needs to be changed on top of this. At least the chip. The programming is engine, transmission, vehicle specific. Any change to displacement, power output, RPM range, vehicle weight, the relationship of weight to power, gear ratio, tire size, --- you name it---- and the relationships of information received from the sensors no longer matches the program maps for fuel flow, ignition timing, gear selection and transmission operating pressures. So without a new chip all these internal relationships become wrong for the operating conditions.

Bogie