I just came across your post - I know you posted in July, so you may have already resolved the problem. Were you able to obtain the measurements you were looking for? Reason I ask is that I too have a '77 Excella 500. I have just taken out all the curtains in the front and am able to measure the tracks for you and/or post some photos.
Let me know if you still need any help with this issue.
1997 Airstream Classic Excella 25ft
1995 GMC Suburban 1500 6.5TD, 153,000 miles
3.42 rear end; 3 inch Warpspeed exhaust system
K&N type air intake; Re-located PMD