Ehman
1) TCI makes a TCU - transmission control unit module for 4L60E and 4L80E GM transmissions. TCU allows to retrofit PCM controlled transmission into a early non PCM equipped vehicle. Check TCI web page and part number 377000 for HEI equipped cars,
http://www.tciauto.com/Products/Electronics/
2) I am not sure 100 percent, but 4L60E were built much better than their younger cousins - 700R4. I would check with a reputable transmission shop regarding V6 to V8 4L60E differences. Chevy 4.3 L is a 90 degree V6 engine with standard SBC bell housing pattern. My guess that there might be some minor internal differences, such as number of clutches used in V6 vs. V8 applications. Also, around 1996, a bolt-on bell housing was phased in (along with a six-bolt tail housing) when the transmission was bolted behind an in-line four cylinder or the Vortec engine family. However, do not take my word check it.
One final word - speedometer connection. In the 90's GM went to all electronic speedos.
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"The Beast"
1975 Chevrolet C20 longbed
350/
700R4! with 3inch body lift
Dual Flowmasters Super
40's!
TBI retrofit completed (2007-07-29)

New 383CID (+030) 08-304-8 9.5:1CR x36,005 (2012-12-17)