V6 to V8 engine mount conversion...
Hooker now makes some really nice engine adapter mounts and are available through Jegs and Summit.
Hooker 12611 - Hooker Headers LS1 Engine Swap Headers - JEGS
I've heard of guys actually turning their adapter plates around backwards (triangular section of mount pointing towards transmission) to mount the GENIII engine into a V6 frame. I prefer to convert the frame to V8 mounts.
Parts needed...
Buy these part to fit your year model truck.
V8 frame mounted rubber engine mounts that bolt to the crossmember(pre-1999)
V8 engine mount cups that bolt to the side of the engine block (pre-1999) 15719194 (thanks Brian1987)
If you were unfortunate enough to have the V6 engine before, now you get to have some fun!
To make your frame V8 style you have to remove the V6 engine mounts/pedestals. You should remove the lower control arms from the spindle and unbolt/remove them from the frame along with the coil springs to reach the bolts inside the crossmember.
Once you have unbolted the factory V6 mounts from the frame, it's time to install the new V8 engine isolator mounts. Bolt the new rubber isolated engine mounts to the holes in the crossmember. You'll have two holes at the bottom and one at the top of the mount. Use the bolts from the V6 mounts. Once you have the conversion to V8 mounts done you can replace the coil springs and lower control arms.
It's a great time to replace the stock coils with lowering coils or to cut a full round off of them to lower the front about 2.5".