Join Date: Sep 2014
The 86 - 91 won't swap in, not without a lot of mod work. For one, you'll have to figure out something for the heater/defroster controls, because the 86 - 91 cluster has no provision for them, those controls moved to just above the radio. The under-dash supporting structure isn't there either.
You can literally swap in any year dash pad, but you might have to paint, or do some other type of coverup work if you swap in a Cherokee/J-truck dash pad. I think you'll be ok with the 79 to 85 swap.
For the drivetrain, front axle would need to be done, you might have to rework some of the exhaust to properly clear the new driveshaft, etc., the QT switch in the glove box is the same as the 2wd - 4wd switch on the 85. The low range - high range lever will work the same, shouldn't be much of an issue. Is the 85 front axle a disconnect axle? I forget, but if it is, you can lock it into place and get around that little issue, then you'll have shift on the fly, like later FSJs.
The largest tire you can fit would be a 31" without installing a lift.
Mileage?? Power?? Seriously, let it breathe is the correct answer. You're good on the exhaust manifold (headers won't help that much, certainly not for what they'll cost), but the exhaust pipes can be opened up some, a better flowing cat and muffler will also help. You've already switched to a better manifold and carb, so the next upgrade would be a better cam, the Edelbrock Performer is a good all-round choice. A better ignition (either the Ford TFI or a DUI dizzy) will be a big help, it'll allow you to open the spark plug gap some. Check and re-check your timing, IIRC, 8* BTDC is what you want. FI of some sort won't get you better mileage, but it'll help on driveablility and off-camber stuff.
His: 1999 TJ Sport
Hers: 1987 Grand Wagoneer