I have a 98 Blazer 4X4 with a 4.3 vortec, and its leaking coolant into the oil. I also have a 94 Astro all wheel drive with a 4.3 vortec, the motor from the Astro is a new GM the crete motor with 50,000km on it and the Blazer has 250,000km can i put the 94 into my 98 Blazer, as the two motors are different and what do I have to do to put it into my Blazer. Thanks

Short answer...no. Although they are both 4.3's, the '96 and up were changed dramaticly and will simply not work. You need to find a '96 up engine.

thanks for replying. Would I be able to use the motor if i got the wire harness and computer the Astro van, and will it bolt up to the trans.

I have a 98 Blazer 4X4 with a 4.3 vortec, and its leaking coolant into the oil. I also have a 94 Astro all wheel drive with a 4.3 vortec, the motor from the Astro is a new GM the crete motor with 50,000km on it and the Blazer has 250,000km can i put the 94 into my 98 Blazer, as the two motors are different and what do I have to do to put it into my Blazer. Thanks

A little longer answer: If you use the heads and intake from the 98, the 94 block will work. if....
* You have to use a new timing cover for a 98 to fit the crank sensor, which th 94 does not have. If it is a late 94 or newer blockthe timing cover will seal OK. If it's an older block you might have sealing prob on timing cover.
** Also be aware the crankshaft must be from late 94 or newer to allow proper fit of the crank-sensor toothed wheel.
Read the long atricle here: http://www.automotiveforums.com/vbulletin/showthread.php?t=534327

It's possible, but if your going to that much work you should just fix the 98.