Running a 65 TH400 the early design with a box filter. Good overall condition and just installed a Transgo 1-2 kit. After install the trans shifts great in "D". Going into reverse it is slow to take gear and seems weak but doesn't slip once in gear. The other issue is when in L1 it feels like it's in 1 & 2. It pulls but has a noticeable drag. What could I have installed wrong when installing a shift kit to have the symptoms like this?

 

It sounds like the TransGo separator plate (sandwich style) may be leaking between the layers...

It is possible for reverse to engage softer, even if the plate is good.
This plate dual-feeds the direct clutch; making reverse apply softer/ slower if the feed orifice is not enlarged (especially if the 3rd accumulator is not deleted).

However it sounds like the separator plate may be leaking between the layers causing another clutch to drag in L1.

I would try to re-install the original plate or a new stock type plate.