Frame diagram(s) don't show brace plates under steering head which all road bikes had.
There was a lot of discussion recently about whether slimlines had 24 degree steering angle, or whether ALL featherbeds had 26 degree steering angle....
Fitting a wideline tank to a modified slimline frame is not going to give you the reduced width in the knee area that was the reason for going slimline ??
When you count them all up, there were about 20 versions of featherbeds, so who knows "how accurate the manufacture by Reynolds" was. ? You certainly hear stories of them having a hard life. With all the replicas these days, must be awash with versions, take your pick...