as said above. It is imperative you get an AX15 trans to fit behind the 4.0, which I believe was 94-95. The weak AX5 just will not last too long, even in a four banger. They were just too lightweight of transmissions. Novac is a company that makes adapters and has some informative information on just such a swap. I read there that the AX15 will even handle a Small Block Chev in moderate build or stock form. Likely Stock form would be best. With the 4.0 making 190 hp? you could figure a Small Block making 250-300 hp would still be safe. But go rock crawling and bogging and you will soon find the weak points in the trans, transfer and axles & drive shafts. A 4.6 or 4.9 stroker will likely be a nice swap. The thing is, a 4.0 and trans weighs in, if I'm not mistaken @ 500lbs. A SBC comes in around 450lbs.
literally just did this swap about a week ago. There is a bit of fab work involved and like said above you need motor back. If you want more power I wouldn't waste time on the 4 either find a 6 or v8. I don't know where your located but there are quite few jeep junk yards you can get the motor back with wiring for a reasonable price.
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