Not a 4l65e but I installed a 4L60E behind a small block. It is doable, not cheap but doable. I had to do a bit of hack at first to get the torgue converter to bolt up (I used a LS spec 4l60E). I had to have the flex plate drilled for the bolt pattern of the LS style converter and had to use a TCI spacer between the trans and motor. Finally, I was able to get an actual GM converter for it but I plan on using it on the next swap as the other one is working fine and no reason to take it apart.
There is an actual GM performance kit to do the swap but its never in stock and they don't list the part number for the flex plate that fits an LS style trans and small block motor.
The swap is expensive and as I have 2 trucks that are almost identical as far as powertrain, lift, axle ratio, and tire size I can tell you the swap was worth it. I like it enough I plan on doing the other truck.
This is the spacer I needed. You will also need the lower dust cover which is the cover from a standard transmission.
https://www.summitracing.com/parts/icb-551835-375fl
This is V1 of how I did it, maybe not right and real trans pros may be screaming at their screen over it. My next run at it I plan on having the correct flex plate/torque converter.
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