I found replacement downpipe studs ( 4 totoal ) at Welch Jaguar Parts out of Ohio real cheap and quick

Tried the large and small hardware store bins of 20 different stud sizes to no avail

That threading of the 2 small solid lines into the sides of the exhaust manifold got me , so may want to start them in free air before moving exhaust 2 castings into their position

True on the cooling of the ignition coils

California has a different engine ECU that may tweek the exhaust products for their emissions , at least on the TSB 493 EGR upgrading including the replacing upgraded PROMS ( data array maps )

 

From old school thought , the proper way to set up for crack welding is to build a heavy and I mean boiler plate fixture and bolt on manifolds , Place whole bolted on fixture in a preheating oven then weld , the cool down procedure is to place back in oven

Now the challenge is finding someone that does that procedure

The yard I pull parts from has a X300 that may have downpipes , only visual if they are or not cracked