how to install one in a manx do i need an extra bar and tabs

You are going to need to fit a strong tube across the back of the seats. I'd recommend a tube at least as strong as your roll over bar's main hoop.

Do not fit them to the fibreglass behind the seats as that is not nearly strong enough, unless to build a strong steel hoop that's connected to the chassis or roll bar under there.

You will need welded on tabs that are at least 5mm thick. The common engineering specification for bolt holes is that the hole must be at least 1.5 times the diameter of the hole from all edges of the metal.

If the tabs are going to be a bit too close to the back of your neck if they are welded to the front of the bar, then fit them to the rear of the bar. Remember that all this tubing and tabs should be padded and it's all going to be very close to your neck.

The specification for the strength of a seat belt fitting is that it must carry 16 times the static load, or the weight of the person, as a shock load. Do the maths. That's a very big number, so make sure that you make it all very strong.

Is it a registered vehicle? If so and if memory serves me well, the Australian Design Rule for seat belt bolts is for 7/16 UNC grade 8 bolts. That's the only place where a non-metric bolt is still used on an otherwise metric car.

Also the nut or the bolt must be captive, so that you need to use one tool only to fit and remove the seat belts.

Because the nuts must also be Nylock nuts, you will need to weld the bolts to the frame.

I cannot get my computer to read my ADRs disc, so I'm not certain of those specifications.

thanks bud all that info was great many thanks

You need to do is consult a engineer on the mounting points for the harnesses as they are a structral point which will require a blue plate in queensland, as for the nut being capsulated that will be a requirement for a production car and to make it convenient to fit the belts in a inaccessable place, the sabelt harnesses that i have are ADR approved and have a high tensile mounting ring that you can unclip so you can remove the harnesses from the car for cleaning, these are supplied with the matching 7/16" unf nuts and spring washers, not nylocs. if you weld any high tensile bolt or nut you will heat treat it and it will no longer be the grade that is required to mount the belts. capsulated nuts will not be a requirement to fit the belts to your car