How much space do you have behind the bulkhead that separates the passenger compartment from the drivetrain compartment? Measure from the current axle centerline to the firewall. Then measure the engine/transaxle package that you want to use.
I seem to recall that Kelmarks were designed for VW chassis & drivetrain. May not be enough space behind the seats. Remember that you'll want enough room in front of the engine to reach the spark plugs and not have the exhaust heating up the firewall behind your elbow.
Also, measure the total width of the rear suspension from axle flange to axle flange & compare that with the one you want to install. If it's wide enough to make the tires hit the fenders you might be tempted to shorten the half shafts, and that's where it becomes outrageously expensive. You prob don't want to install, for example a Buick V6 into a cradle with Volkswagen suspension. you'd want to use the entire Buick setup just as Buick engineers designed it. That way you minimize the fabrication and the Buick axles are designed to take the power that the Buick can make.
Alternatively, you might be tempted to widen the rear fenders. This would be easier and, if done carefully, might make for some major styling points - like a Kelmark on steroids.
On ChevyTalk.com there is a BBS for Corvairs enthusiasts. In there you'll find people who have converted Corvairs from rear engine to mid engine. Some of them may be able to give you useful advice. Also Google for Corvair V8, corv-8, mid-engined Corvair & stuff like that.
check these out:
http://www.pro-touring.com/featured_...west_corv8.htm
http://www.automotive-links.com/mak/che/vai.htm
The most important things you need to know about midengined cars concern the shift linkage and the cooling. The shifting is easiest to handle by using an automatic with control cables. If you want a standard then I wish you luck.
If you're going from a VW or Corvair ingine to a water-cooled engine then you'll have to fabricate an entire cooling system. If the Kelmark has a very low nose (I can't recall right now) then you will want to consider a Corvette radiator that sits at an extreme angle. You'll prob fabricate duct work to channel the air from the grill to one side of the rad and from the other side to outlet vents behind the front wheels or in front of the windshield. You'll have to run stainless steel tubes from the engine to the rad, either thru the center tunnel or under the rockers below the doors.
Otherwise you might consider a low, wide radiator in the back with a few electric fans pushing air thru it. This will require venting the transom and you'll have to wrap the entire exhaust in insulation to keep it from heating your radiator.
One of the biggest complaints from folks who have built midengined Corvairs has always been about cooling. Remember that your cooling needs are directly proportional to your horsepower level.