The front of an inline 6 is about the same position as the small V8. That means the bellhousing on a 6 cylinder truck is closer to the rearend.
The engine and transmission are mounted more amid-ship on the inliner. Putting a V8 in the 6 cylinder position makes for better weight distribution IMO. You can look at pics of other trucks with HEI to see how far they are from the firewall to see how yours "measures up"
After Googling images of HEI installs, yours looks like everyone elses! So probably my imagination.