Sible,
My brother used hedman headders (cant remember the size probably 3 inch I think I have a pair of 3.5 inch or 2.5 inch here that they never used boxed) from Real Steel and lots of 3 inch sections to make up a pattern exhaust. Then went to a local performance place that bent up 2 exhaust systems minus turbo mufflers these again got from Real Steel for the 90. I will ask but they were like only £40/50 each exhaust and the muffler was only like 30 quid. The hard part was making the pattern with plenty of room around it and keeping it up protected out of harms way. Fastest 90 I've ever driven (only 15 at the time in my brothers workshop) took many sessions on a rolling road to get the 3.5L jetted correctly but once it was it was soo darn good. I'll ask for the final read out from the rolling road sessions it was some stupid torque figure enough for the engineers to question the actual CC twice.
The beauty about the 3.5 is that you can compete in upto 3.5L class in some comps and if you have a well sorted engine you could out perform the average rebuilt 3.9's and some of the later larger engines just by getting the engine components matched and working together.
This engine was that good that one of the main national scruteneers for the comps we do had to have it for his competition motor. We took the rocker covers off after ?K miles and the oil was still golden and like new.
Rover V8 is my brothers thing much like guy that did your engine I suspect. The work that went into it was just no compromise. The engine caused a bit of havoc as once it was fitted to the trials vehicle that the scruteneer had I had people pestering me for his contact details when ever I went trialling. Nobody could buy this type of rebuild he would not do it for anyone. It was built with care and putting bits that work rarther than the usual (I got a TVR engine, well heads, and cam bollix) it was an awsome engine completly built for the 90 new. My bro trained with LR upto and including RR efi, Had his own buisiness for many years, trialed, raced and built offroad spaceframed specials for competition for many years in his workshop aswell as the usual service stuff. I helped after school and was brought up learning as you do usually getting diff oils and abused as you do in any workshop.
Engine building is about matching parts that perform well together. Bolting on bits especially to an older engine circa 100K depending on its previous use/abuse isn't always the best idea. At 100K It will probably need a camshaft anyhow and you may aswell rebuild it properly if your tearing it apart.
My bro is talking about doing an engine rebuild again soon making the V8 more viable affordable for everyday use aswell having all the performance and more by making it far more efficient. Watch this space. (maybe a little while tho but it will happen there is a list of motors to re chassis rebuild as new first)
I have a 3.9 in my 80 inch which no doubt will have been pulled out a RRC and messed with. Apparently has had a cam I just hope its not some stupid person fitting a race cam and I hope they havent replaced the lifters with some high spec ones cos it might end up a rocket ship with bugger all low down oomph.