Are these the stock 420 manifolds?

http://cgi.ebay.com/ebaymotors/454-B...QQcmdZViewItem

No. The stock 420 manifolds were the worst, were quickly discontinued and replaced with the Merc/Gil replacements.

Those manifolds on Ebay look like cast-iron, aftermarket replacements by Barr/Osco. They are a step backward from the ones you have.

Unfortunately the 320 Baja engine room design gives poor access for servicing no matter what exhaust you have. Years ago I had a customer that had that boat and I dreaded working on it for that reason. If you want more power go to Stainless Marine center outlet 4". Lighter than cast iron and flows like crazy compared to the exhaust you have now. The sound will please you too! Good Luck.

Those actually look like like KLM manifolds. They were an aluminum replacement for the stock Merc cast iron. They claim to flow a little better than the stock ones. They still uses the same type of riser with waterpassing thru a gasket.

You could be correct about that. I'm surprised the seller wouldn't play-up the fact they are aluminum if they are. I would think that would be a big selling point.

I thought maybe the stock 420 would be better then what I have, I will use what I have until later I guess, I am doing alot of other work this winter so I whole "new" exhaust is not going to happen right now.

Just remember that a better breathing exhaust system like Stainless Marine or GIL, etc, is (or should be) the first step while doing engine mods/upgrades when looking for more power. Have Fun.

Tests run by a performance boat builder and ex racer with access to Dino said that it made no difference in HP using 3" or 4" which was a contradiction to my land racing.??
Logic would say that 3" would be OK if you dumped water near end wheree it would expand from heat and increase the volume being exhausted. Ala water cooled pipes as found on some race boats.
My experience with Stainless pipes,mostly Stelling, was that they recommend pressure testing every 20 hours. PEW!
We did test some and found numeerous pin hole leaks on the interior welds . One set had 8 leaks and were sent back to the factory because of low hours. They "cut holes and patched" ! this was on a 50 foot Nortech.
Bottom line; I would be carefull using the SS tube in a tube headers. ...And don't forget IF you lose water pressure for any reason it is going to stress the SS.
Walt

One more thing. Check the distance betwen the engines and see if you have enough room for cast headers. Normally the clearence is less than an inch. header to header in boats under 39'. They should give you a C-C motor minimum dimension for the headeers. They may have used SS to squeek the motors together.
Just a thought.
Walt