I measured mine this morning, the front of the lens is significantly smaller than 5.75", more like 5". But is you're talking about the OD of the ring they are mounted on it looks pretty close to 5.75 (Sorry I wasn't going to take mine apart to measure). The 1st generation (86-87 750s, 86-88 1100s) have a slightly different headlight setup so if the above doesn't fit your needs you can look into those. Hope this helps, drop me an email if you've got any other questions.
Matt