If it was me i would go 12 valve with a beefed up 4l80e that trans will fit the truck fine,As far as a t case a 205 is kinda king with out going aftermarket.There is steering box braces which are a must to me and there is a rockram which is a hydro ram on the steering kinda like power steering for big tires.For led tail lights look at
http://www.customclassic.com/ his has more leds and can have the reverse in led too.For the cummins swap
http://www.avalancheengineering.com/xmember73-91.html or
http://www.autoworldmt.com/Page_11.html two sites for everything else
http://www.destroked.com/ and
http://www.tndieselpower.com/site/products-page/ has some nice stuff like a mid mount ac .