KG Trimning is the other reputable company that sells 4-2-1 header systems. It comes in 3 flavours of tube sizing depending on your level of tune. I think VP Auto sells or used to sell the KG Trimning headers.

Hi Performance Auto in Torrance CA can source products from VP Auto. Depending on where you are located, they may be able to supply the exhaust system at a lower cost. The Simmons sport exhaust system is robust = weighs a tonne. Make sure to check shipping costs when evaluating the total cost of the package.

Tinus Tuning seems to have a good reputation as a reliable supplier. I know of at least one owner with the TT3 cam who is very satisfied. Tinus Tuning has the advantage that if you email Ben and tell him what you have / want he can make product recommendations.

If you are looking for a head, rumour has it that the F head make actually be a better piece to start out with than the E head. Rumour has it that exhaust ports are slightly different and don't have quite the nasty bend on the short side radius (as in better , not necessarily good). I have never had an F head in my hands so I can't verify that. If you have significant plans, an F head which has the same larger valves as the E might be a better starting point. Where I live, vintage Volvo parts are rare / non existent. You could check or post in the wanted section of Turbobricks to see if there are any kicking around. You could also try Planetman (Hi Performance Auto) since they have a trove of salvage parts.

CVR lists B20 heads for sale.
https://classicvolvorestoration.com/category/classic-volvo-parts/volvo-engine-parts/volvo-b20-engine-parts/volvo-p1800-b20-engine/
I don't know whether those are restored or just salvaged. If it is a restored head and it is available from their North American warehouse that might not be a bad price. If it has to ship from Europe that will be ouch.