The V8 manifold bolts are a problem, they do work loose, but the reason is obscure unless you understand fastener technology. For a bolt to stay tight it must stretch when torqued, the bolt should also be elastic enough to maintain tightness after the manifold has heated up and cooled down. The problem here is that the bolts are not really long enough to accommodate the amount of stretch needed and after some heating / cooling cycles they stretch beyond their elastic limit and loosen. Tony Lake made a suggestion to me that a longer bolt with a spacer under the bolt head may be a solution and I have made some 3/4" long spacers and fitted them to the manifold bolts my RV8 on the LH bank. After half a season's hill climb, sprint and track days the one with the spacers under their heads have not required tightening, but the others have. This isn't a total solution because on the RV8 there are some bolts that do not have clearance between the tubes or adjacent parts. I haven't been able to check a BGTV8 with Cast iron manifolds yet.
I'm currently investigating other solutions, but have not had time to delve deeply. IMORTANT!!!!! Do not overtighten the manifold bolts, they are steel threaded into an aluminium head and cannot stand too much torque. If you do you risk scrapping the head, because there isn't enough metal between the bolt hole lands and the water passages. The recommended Torque for B GT V8 is 13 lbs Ft [1.80 Kg M] and RV8 40 Nm [29 lbs ft.] This seems a little high, but the RV8 has a fabricated manifold rather cast iron.
If the manifold has been loose for some time it might be worth checking that the gaskets are not burned through. If they are replace them as soon as you can to avoid damaging the head, Clive Wheatley has them.