283 man is right, your best option for upgrading parts is any newer truck up to about 1987, Chevrolet re-designed their trucks from the ground up for 1998.
I believe your truck will actually already have what it needs to mount a small block into it. If you take your 250 6cyl out you will notice the motor mounts with the rubber vibration pad (connected to the motor) will unbolt from the "spacer" motor mount that the trucks use to support the 6 cylinder motor. By removing this "spacer piece and using the single bolt to mount the rubber mount (that was attached to the motor) directly to the arched motor mount perches (that bolts from the frame rail to the cross memeber...) Your done.
As for the transmission, you will need to cut out the existing cross members that support the tranny (unless its already and automatic truck, in which case you just need to move the member back) find a 80's chevrolet Blazer tranny cross member and get creative. watch your driveshaft angle! Oh yeah, you will need to get a new driveshaft made. I used one from an early 80's long box chevy truck (larger size u-joint) cut it, welded it, balanced it and then used a u-joint that had two different sizes of bearing caps so it would mount up to the existing smaller size rear end yoke.
Good luck!
probably be good idea get new motor mounts.