VERY tough LT1 was never offered in that truck so you have to use "trail and error" to find motor mounts, radiator, hoses, computer and wiring harness to make it run plus the V6 transmission wont hold up to the power of this V8
A better option is to buy all the parts from a wrecked 88'-'95 pickup or SUV and transplant everything This way you'll have a V8 and wont have to swap the dash, guages,and updated computer controls
If you want a newer engine like the Vortec '96-'98, You'll need the whole wrecked truck and swap the guages, DLC(OBDII scanner connecter) the dash and wiring, electronic transmission, and ABS module and sensors, including the rear axle for the traction control/ABS
Like I said VERY tough Stick with the '88-'95 and you'll buy replacement parts for a '90 with a V8