Indianapolis police officer here.

A 1978 originally came with a VIN on the dash, although it might not conform to modern VIN standards. If its not there the dash has been swapped out or the previous owner removed it for some reason. Newer vehicles have hidden VINS throughout to assist in recovering stolen vehicles and parts, but the '78 doesn't have them.

To be legal in Indiana you need to get a VIN on your dash. Simply buying another truck and pretending its that one can get you in trouble. Even if you could do that, just not having a visible VIN is immediate grounds for impounding the vehicle to verify the VIN and if the vehicle is stolen.

You can go to the BMV and apply for an "M-VIN" number, or manufactured VIN number. Since the truck is assembled from multiple trucks and the true original VIN is unknown, Indiana will assign you one. They will also make a VIN plate, slightly larger than a true factory one, that starts with an "M" and has a unique VIN number. The BMV will affix it with the proper rivets to your dash. From then on you will be legal and can transfer ownership, drive on public roads, etc. just as you could with a factory VIN plate.

*edit*

Here is the IC code relating to MVINs and how to apply for an identification number. Note that, as I stated above, the inability for a police officer to verify the VIN from the exterior of the vehicle is immediate grounds for impoundement of up to 48 hours to verify the vehicle's status.

http://www.state.in.us/legislative/i.../ar18/ch8.html

Also note that it is a C felony to remove a VIN and a D felony to sell a vehicle with the VIN removed.