Here's what I found.
The Lionel Super 381 has a maximum width of 5-3/8"
In my modification to the Pride Lines canopy, I used pieces of brass 1/8" x 3/4" x 1-1/2": the holes drilled are spaced exactly 1" on center, which gives an extra inch of width on each side, for a total increased width of a full 2". This is propbably more than you would need to do, but I needed it because of the curves right at the end of the canopy.
AS MODIFIED, the width clearance of my canopy is now 6-15/16" between the posts, just a hair under 7". The height clearance is also 6-15/16 from the top of the tubular tinplate track rail.
Steve can verify this by measuring his unmodified canopy, but since I gained 2" with my brass pieces, the width clearance of the canopy unmodified should be right around 4-15/16" or 5".
Since the Super 381 is 5-3/8" wide, it would probably not fit between the posts of the canopy as it came from Pride Lines. I say "probably" because I am measuring the width clearance between the largest diameter part of the turned post, which is at the top and bottom. But the widest part of the Super 381 is the cab side railings which are more in the center of the posts, where the posts are thinner. It might actually just squeak through.
As for height, the Super 381 has 3/16" to spare over its roof. Needless to say, this is with the pantographs down. Because of the 1/8" thick brass pieces, I gained an extra 1/8" of height clearance. So theoretically the Super 381 would still clear by 1/16" under an unmodified canopy, but it would be very close and YMMV.
david