I'm definately keen on building a full system.
Do you know what pipe diameter you used for primary and secondary pipes.
I'll be looking for a lot of info about what works and any formulas for calculating diameters and lengths.
Did you just use 10 inches 3" strait tube after the collector then reduce into the 2.25" or was it like a megaphone ?
I can't recall what size the primary pipes were but it started life as a brand new set of HM headers for stock 1600, smallest pipes I could find when everyone else was going big bore, special order. Flange and gasket were mated to the head and exhaust ports like you did.
Secondary pipes were cut off and the junctions on the inside of the primary pipes were welded to fill any gaps and then hit with the die grinder to clean them up - fixed the "out of sight out of mind issues" since the manufacturer only welds on the outside of the pipes.
New secondaries were each 3ft of 2 inch, needs to be bent up once the primaries are fitted to the engine in the car to get around the steering gear and firewall.
"Collector" was just a 10 inch piece of 3 inch to bring the 2 secondaries together. The was no special fittings or anything, the exhaust guy just made it all fit. I was told after that I could do anything I liked with the rest of the system, within reason.
To suit rallying I had 2.5 inch for the rest and had a slip joint about a foot or two back from the collector and a straight through "turbo" muffler after the diff. Pipe went over the half-shaft and muffler mounted diagonally under the boot floor and the exit on the right (instead of left like standard). No spare wheel well.
2.25 inch pipe would have done for anything but rallying, the 2.5 inch allowed for some crush and damage.
Formulas unknown, I was told exactly what to do by a experienced educated engine builder and long time rally freak that had spent a lot of time on the engine dyno and then chassis dyno with the L20b with twin 45 Webbers over a few years.