Hi Muskrat, folks.
A few pics of the barrel which is bored to 71mm, the original (cylinder head) tapped holes were plugged in 4 positions using aluminium, an odd choice? then re drilled all the way through the base 0.377"dia, compared with an original barrel stud hole 0.332". The pitch across the barrel is 6 9/16" the pitch front to back is 2 7/16" I have measured this with a steel rule only, there is a discrepancy of 0.020" on the F/B pitch but it appears true and a pretty good job.
I will simply use the barrel as a drill jig and spot through, into the crank case/head. In order to use 3/8" Dia studs I must open up the existing barrel through holes by at least a further 0.020" for clearance, not how it was envisaged in the original mod. If I use 5/16" studs, well it feels a bit small dia when it will need to be approx 5 1/2" long plus thread. This is why I would like to find a suitable long bolt used in a similar application to then modify, though the problem could then be that such a bolt requires rolled threads. Dunno.
Interested in any thoughts
Chris P