I'll try this again ,1 hr. and my thread dissappeared when i selected post .
I have a 94' Ford 4x4 Bronco with a new re-man 351W longblock with 5k on it . Running stock exhaust , 3.55's and 31-10.5-15's ,14* initial timing ,stock ignition . I've lost close to 100 miles range on mileage .
Old engine was 155-165 psi. across the board except number 5 which died , new engine is 135-140psi.
I've lost Torque/Perf/Mileage severly .
I was thinking (dangerous territory here)about the fact the bores are bigger and that the stock cam is not allowing a full charge to the cylinders .
I believe this is the OBD-2 system ? I've heard that there is very little that can be done without sending the ECM into convulsions ,correct ?
I hollowed out the Catalytic Converters and resealed them (NO Smog Laws-but has to look intact) .
Now my truck smells rich at an idle .It also falls flat on it's face when passing at about 75mph . 90 mph used to be no problem .
I'd like to upgrade the exhaust with Summit Shorties into the stock Y-pipe , a Magnaflow 3" Spun Metallic Cat. Box into a Magnaflow or Dynomax 3" in and 2.5" out single .A cam that came in around 1800rpm would be Ideal . But I've been told that it may not work due to the OBD-2 system on it .
We live in a remote area of the Missouri Ozarks and this truck is our Emergency Hauler/Puller/High Water Crosser .
I live on a fixed pension so GT40 heads etc. are NOT an option .It's nearly $100.00 to fill up now , 2 mo. ago it was $60.00 roughly .
What can be done to get back it's old performance without tearing the engine down ? Is there a Cam/Exhaust/Ignition package that would help ?
I used to get very close to 400 mls. out of my 26gl. tank now it's more like 280-300 if I'm lucky .
Thanks,
Perplexed in Missouri  

A bit more info :
The catalytic converters were melted at the Y-pipe one . They had 163K miles of hard driving .I was told the rich smell may be from the O2 Sensor sensing a Lean Condition due to decreased back pressure after I removed the guts .
The stock exhaust is 2.25in. Y-pipe into the 1st Cat-Box , and 3" out to the large Cat-Box , out to 2.5" to the muffler and 2.25in. tail pipe .
So I was wondering about a higher lift/duration Roller Cam .
At 70 I'm taching at 2k rpm.on the road . If I'm in 4 wheel drive and hauling up out of our valley or hauling out stumps , gravel etc. I'm in the 1700-3000 range depending what range I've selected .
Hope this info helped ,
Steve  

What's your cam specs?  Maybe your intake valve opening should be earlier to build more cylinder pressure.Can you adjust your timing at all with your ecm?You can custom calibrate your ecm.If your timing chain can be advanced lets say the typical 4 degrees that would give you more cylinder pressure and more low end grunt.With your weight and gears I would say no more than about 204 intake duration at .050 measurement. Keep us posted    WIEDER