spacer kit that if i recall north west fab sells . then bolt on and go . https://www.northwestfab.com/4l80E-to-TH400-NP205-AdapterSpacer-Kit_p_3452.html reason is the index ring is 2 different sizes and the 4l80 need room for the pan and a th400 adapter dont have the mount point set back to clear it .The truck looks great!
Search this site, I believe you can use a 85+ 400/205 adapter with minor machine work.
and i ran a lokar trans mount shifter threw the stock 4 speed hole and boot on my old k30 . so many people were sure it was a 4 speed but it was auto . got this model in black and piston for shifter on top like this one . https://www.summitracing.com/parts/LOK-XATS6350AN