The easiest way to do a V-8 would be to use a "donor" truck for the required parts. you'll need a pick up V-8 bellhousing,flywheel and clutch,starter,exhaust pipe and manifolds,front motor mount(the U-shaped part),a new rubber mount,probably new radiator hoses,the V-8 radiator if you can get it,the fan and shroud,also if you can get 'em,the throttle arm (on the firewall)and linkage,and it wouldn't be a bad idea to grab the engine wiring harness from the V-8 donor truck.It plugs into the firewall towards the passenger side of center,and contains the wiring for the starter,gauuge senders and ignition. Some of this you'll be able to get new/recon from your local parts store,some you might be able to use from your six,most you can get from the old truck outfits you see advertised here.
Once you have the conversion parts,you're set;pretty much any smallblock will bolt right in. (If you get your front mounting bracket from a 1-1/2 or bigger truck it's even possible to bolt in a bigblock.They often have holes to fit small and bigblocks.) The point is,the more parts you can use from a V-8 powered truck,the easier it is to swap. There's no fabrication to do except you need to cut a piece of angle iron and drill it to bolt your clutch slave cylinder to the 2 small bolts on the front edge of the bellhousing,right by the oil filter.You can either coil up your hydraulic line and leave it connected or remove it and have a shorter one built,or,again,use the one from the donor truck.
No big deal-when I did my first one I didn't have access to the correct front mount,so I used an old Hurst mount with flathead V-8 biscuits. Hadda drill holes to make that work though.
Speed
