I have been following this forum for about 2 1/2 years now, but this is my 1st post. (I hope I get it right ) Years ago I had an experience with this type of frame that I think you should hear about. I bought my 1st Norton (1st motorcycle) in March of '73. It was titled as a '70, but I was later told by another dealer that the bike was actually a '69 that had been retitled as a 70? Whatever year it was , doesn't matter, it had the old style frame. One morning while I had the gas tank off checking for loose electrical connections in the coil/condensor area, I saw that the main tube and both down tubes were cracked about 3/4 around just like in the previous photo. I was shocked as I recalled the previous night's ride home where I saw the speedometer read 115 mph! I have often wondered why I am still here to tell this story. I had the frame welded by a competent repair shop and continued to ride it, however, I constantly watched for more cracking. About 2000 miles after the repair, It began cracking again , at all 3 places, about 1/4 inch behind the repair welds. I had it welded again, and then traded it in on a brand new '75 Ducati. The dealer was informed about the cracked frame. About 5 yrs. later, by coincidence, I met the owner that bought the bike after I had traded it in. He told me about the day the front end broke completely off while he was going 30 mph! You wouldn't get me to ride on that frame.
I think that you would be best off changing the frame with the correct replacement, and keep the 1 you have "as is" in the interest of originality. There has to be plenty of them out there, if you can find them. As a matter of fact, I have 1 in my chicken house. The number stamped on the frame tag is 137886. It is not my frame to sell, and I have not spoken to the owner since reading your post, but I would be willing to bet that it could be purchased for a reasonable price. ( probably much less than having yours modified) Other than being rusty, it looks to be in very good shape. If this would be of interest to you , let me know. I would then talk to the owner to see what he thinks. If he is willing to sell it, I could then send photos and proceed from there.
Mike