if you are switching to blazer brakes (F/R) you will want the blazer master and probably the booster. the 94 will have a steel brake pedal/steering column bracket you can use. you will ALSO want to save all the brake lines from the blazer, switching to blazer brakes takes either modified truck hoses OR the complete blazer line setup with hoses. drop blocks will interchange but remember that blazer leafs wont fit the truck frame (you didnt ask, it bears mentioning).
the 2wd blazer rear axle is the same width as the 2wd truck axle, so either one of those and you will need low offset wheels or 2" spacers. the 4x4 blazer axle will bolt right to the truck leafs and driveshaft and you wont need any spacers.
you can use the saddle gas tank from the s15, no need to put in the blazer tank. with the way the rear frame and bed works on an AD truck, if you used the blazer tank it would likely be the most visible thing since the great wall of china.
you will need a controller for a 4L60e, and you wont be able to use the factory ECM from the blazer unless you keep most of the engine controls and even the body computer. they sell stand alone computers for controlling a 4L60e, you will find used 700R4 transmissions are cheaper.
ride height with 3" drop springs and body mounts that are 2" lower will look like this
since you are going to an old 350 I wouldnt recommend using the 94 harness (the only one you would use unless you were swapping in the entire drivetrain and computers of the blazer). an aftermarket harness is going to be less work than paring down the 94. again, do NOT use the blazer harness unless you are keeping EVERYTHING, the blazer steering column, the blazer tank, everything.
When I did my 03 s10 4.3 swap and kept EVERYTHING I found a 98 tahoe AC/Heat box plugs right into the s10 heater controls and just barely fits in the dash. the S10 ac stuff is all mounted outside the firewall and wont fit on the firewall of the 3100.
use the 94 steering column, and column mount as I said above. make SURE you grab all the column wiring and brake stuff too like the switch. gas pedal from either will work. maybe. keep them both.
have you read the skymangs sticky thread on s10 swaps? lots of good info there.