If you want a big block,the "horse shoe" mount used on the '62 bigger trucks has the small and bigblock holes already there. Just bolt it to your block,get a '62 front mount pad;measure from the bellhousing mount pads to the front of the horse shoe bracket;you may have to drill holes tor the front pad. I don't remember if there's holes the right distance for a bigblock. Use the '55 V-8 iron side mount bellhousing and whatever flywheel goes with the engine.(in the 454's and 402's cases it has to be for a 454 or 402-make sure the diameter is the same as the 283 flywheel so you can use the old starter.)All the other small and bigblocks (including 348 and 409 "W" blocks)can even use your 283 truck flywheel and 11" clutch. That handles the mounts,exhaust is best custom-built. Without a doubt,your biggest bigblock headache will be steering box clearance. Smallblock-no problem;bigblock-might clear with the right headers,but it'll still be close. The smallblock truck starter will work as is. You might have to get creative to get your exhaust to clear the clutch linkage,but that'll depend on how the pipes are bent.
I'd stick with the smallblock if it was me.
Speed
