They aren't toooo bad to do.
Just remember a couple things (imho)
- forget all that equal length crap, the 1-2% that you lose is NOT worth the headache
- tack welds are your friend... they are easy to dust off and re-do if it doesn't fit quite right
- dont forget once the pipes are run side by side and welded to the flange, you're not gonna be able to get inbetween them to finish that one beed you "left till later"...
Also a trick that I absolutely love (if you use 3 bolt flanges at the reducer/collector):
Use a double flange (two welded together) on the reducer and weld them on so that the entire first half of the flange sticks out above the top edge of the reducer. (about 3/16 - 1/4 inch)
Then when you weld the flange on the collector (the up stream side) slide it up about an 1/8 of an inch onto the collector. (so that about an 1/8 inch of collector is sticking out beyond the downstream side of the flange) The gasket will basically hang on this 1/8 of pipe, when youre bolting the collector flange to the reducer flange.
Now when you bolt em together, the gasket is protected from blowing out, by the little bit of collector that runs inside it, and the double flange on the reducer basically forms a "female" socket for it to fit into.
Blow-proof 3 bolt flange gaskets...
( after their all done, find a local coating place and have em' throw a coat of ceramic on em'. They look bitchin and the best part is, when guy at the shows/meets ask where ya got em' you can tell em you made em... all matter of fact like, as if everybody does it.
)
Dave