The traveller position is about 7" 3" from the gooseneck connection. My boom is 9' 5" to the end fitting existing swivel bracket - so I settled on moving the mainsheet attachment along the boom - and making it a pair of lugs, about 2 foot apart. The forward one shown below is 7' from the mast - then one near the end. The boom end bracket will still service for the topping lift. I've a plan to fit a camcleat on this bracket, so the end of the topping lift can be adjusted either at the boom end, or on the mast cleats.

My ' Bolt holder' prooved a valuable little tool. It needed lengthening and stiffening - a bit of split garden cane did that. Many gynochological jokes came to mind sticking these in !!

So - did it work ?
I wanted to fix the Holt fitting with through bolts - monel rivets did'nt seem up to it somehow. I also needed the bolt head inside, to minimise any fouling of the in-boom outhaul block arrangement. It's also much more feasible to introduce a bolt outwards, than try to locate a nut, plus washer, inside.
If you use a spring locking washer, you only need the ' boltholder' to stick the thing through the hole - once the nut is on it will pull up. At first, it will seem not to hold, but you have to 'jerk' it with a ring spanner - eventually it will hold tight. All six pulled up, holding firm as hard as I can pull on an M5 spanner - thats prob. about 40 ft lb. The washers, one spring, one penny, were liberally slobbed with Duralac, as with the back of the fitting. The bolts were trimmed to just take a nyloc on top of the base nut. You cannot just put on a nyloc - unless you can fathom a way to hold the bolt head inside the boom. I could have done this, possibly with the outer fitting - but this one, about thirty inches along the boom - and with various fittings getting in the way.
The bolt heads 'look' a bit obtrusive - but the rings are large enough for nothing to foul. Altogether, one of those jobs I thought was going to be a right pig - but went as sweetly as could be.