FYI
In 1990, the 700R4 was redesignated as the 4L60 (4-speeds, Longitudinally positioned, 6000 lbs. GVW). This was a change in name only with no significant mechanical changes associated.
By 1984, 700-R4s intended for use behind small-block Chevy V-8s began to receive beefy, 30-spline input shafts similar to those found on classic TH350 and TH400 transmissions. From 1984 to 1987, the most failure-prone internal parts, from the ring-gear to the oil-pump housing, were upgraded. An auxiliary valvebody was added in October 1986. Finally, on performance cars like the Corvette, additional internal lubrication improvements permitted the trans to survive in high-gear, full-throttle, top-end conditions. Many of the improvements can be retrofitted into the earlier, weak transmissions, but the 27-spline versions need so many new parts that it's more cost-effective to swap in a later core. However, it is worthwhile to upgrade the later 30-spline versions to Corvette/IROC-level internals, if they're not already so equipped.
Should not be any more work than swapping a 91 4l60 fromt another truck.
