code 740 is torque convertor circuit malfunction and 700 is TCM malfunction.

I dont think you installed the shift kit correctly or maybe pinched and oring in the process or something.

sounds to me like your tranny might either #1 be shot
or your tranny fluid is really low/dirty.

always start tranny problems out by checking the fluid level/condition first, thats super important in our vehicals.
too low on the fluid and you get slippage.

check the fluid first, then I would reinstall the shift kit.
did you already drill out the holes,etc..

what I think is happening is that the fluid level is incorrect or the gears are slipping from too much wear/heat.

Your on your own with this one, sounds like time for a rebuild.

but yes check fluid level first.
I would actually get it flushed/refilled