You can install a 350 in place of the 250 using your original trans and bellhousing,you can even use the flywheel and clutch off the 250 too. You dont have to buy motor mounts,just use the rubber mounts off the six ,get rid of the adapter that bolts the rubber part to the block and bolt them to the 350 and install the motor with the sixes bellhousing and clutch assembly. Its been about twenty years since i did that swap,you will have to get the ball that screws in the block for the clutch z bar and if I can remember correctly if you use the six z bar,you have to grind some off of it on the end near the block to make it fit. The motor will sit back close to the firewall so you must use a LONG STYLE WATERPUMP and a FAN SHROUD and a bigger RADIATOR. Trucks that came with a v8 originally,the motors were moved a little forward. I done this to a 67 c-20 and it worked just fine and there is room for a hei too and you dont have to buy much this way and use rams horn manifolds that exit straight down or it will interfere with the clutch linkage. I hope this helps
