Willie, I usually order from Rock auto, in the states. The parts are a lot cheaper there but the postage and tax will bring it up quite a bit. Although chevy parts are substantially cheaper delivered than parts for motorhome base vehicles here. I think the last water pump I bought was 47.00 all in when it arrived here.
Make sure you select FedEx international or priority and the parts arrive in 3-4 days. If you select consolidated or ups it can take up to 1 month and taxes and vat + fees have to be paid to royal mail before release. With FedEx the taxes and vat are paid to Rockauto and there is nothing further to pay.
Rockauto have quite a few warehouses throughout the states and care needs to be taken when placing multiple orders as various locations means multiple delivery charges. Try to order all from the one warehouse. This can be done by ordering the silencer first and then if you want a fuel filter go to fuel filters and look for one with a lorry symbol beside it. This means from the same warehouse. Adding additional items from the same warehouse that you require now or in the near future makes little increase in the initial delivery charge, so is cost effective.
A truck with a union Jack means that it is available from their UK supplier and is very reasonable delivery, ensure you have your location and sterling settings.
They may not list your coachbuilders individually but will have many big block vehicles you can select from. You will need to check against this for centre in out, staggered in out pipe, staggered and centre in out or 2 pipes in or out. Many silencers are directional inside so check that the in out combination are the correct way around. Finally your pipe size 2, 2 1/2, 3 inch or step up in size on the outside.
Sorry for melting your head