I have stock sending units in on the side of my block. Can't figure out how to replace them with ones that match the auto meter marine gauges I have. I know that the stock aren't compatible cas the gauges peg when the key turns to accessory. Then when the boat fires the motors read 65-70 when cold and at 3000 rpms it drops to 25. After the motors warm up they shoot back up to 65-70 at idle and drop when rpms are up. It's like they are wired backwards. Any chance the may be wired wrong at the gauge. At this point I think I'm going to look for stock gauges rather then try to swap these sending units.

Yep, the gauge sender is working in reverse. There is not really any way to wire tehm backwards to do that. When I got my Pantera, it had a VDO sender going to Gaffrig/Livorsi gauge and it did the same thing. I would call Autometer and see what sender you need.

My AutoMeter Proc Comp Marine gauges work fine with my factory Merc sender, but mayber like Griff mentioned at some point somebody changed your senders out to match a previous set of gauges.

http://www.summitracing.com/parts/ATM-2242/

Per Rex Marine's website, the ACU 900 will work with AutoMeter gauges:
http://www.rexmar.com/page60.html
http://www.rexmar.com/page57.html

I have the new automated gauges just can't figure out how to swap them out. The originals I can't seem to get a wrench on and to be honest there just isn't a lot if room.

Where are the senders at currently, down by the oil pan/oil filter adapter? That is where they usually are, however on my GenV 330 the oil pressure sender was plumbed at the top of the block, back by the distributor. You could always leave your existing ones where they are and put your new senders up top.

You probably need to remove the power steering pump/alternator to access them from forward.

yup
 

once you remove the alternator you have tons of room on a 311