Well I'm working on my eng. swap, but the other day I started thinking about this, even though it's a little to late to ask, Is it legal to swap a v6 or bigger eng. on your truck? I know a lot of people have done it, my concern is that when I have to go to the emissions, my vin # tells them that the truck I drive is a 4 cyl. right? And what if they set the computer for a 4 cyl. and I wont pass the test cause it's actually a V6 running on it. Do I have to get a state patrol inspection on it so that I have a certificate that proves that there is no stolen parts on the truck. But just in case I'm going to hold on to all my papers and certificates for a while. By the way I am having good progress on my swap. I will post some more pictures soon. Thanks for any input.

These are the rules for MASS: (check your laws in your state)

If you own a vehicle built between 1992 and 1996 (for example)
your emissons regs are xxx Hydrocabons, yyy NOx and zzz CO

If you swap in another engine, no matter the type or year, you must pass the exact same standards. So your new engine has to meet the same requirements as your old engine did. If your new engine is much more high tech and newer, you still only have to pass for the standards set by the VIN and frame of the vehicle.

The only exception is: If your car is OBDII, you must preserve OBDII in your swap.

Emissions regs are based on PPM (parts per million. Going to a bigger engine displacement doesnt effect ppm measurements. The engine just has to run as clean burning.