After thinking about this whole issue some more, I came to about the same conclusion Chip did. If you don't closely circumscribe what can be done, you'll start and arms race on engine location that will make any already overly expensive class (that's another topic) even more so.

I think cars using the stock engine should leave it where it came originally, and swapped engines should be installed as close to the standard location as feasible.

How about this language for the engine location rule?

Engine location (RWD vehicles)-

1. Cars which utilize the engine that was originally in that chassis shall retain the stock engine location.

2. Cars which utilize an alternate engine shall install the alternate as close to the original engine location as possible.
a) The engine's fore and aft location shall be no further back than one of the
following:
1) The alternate engine's mating surface with the transmission shall be no
further back than that of the engine originally installed in the chassis, OR
2) the alternate engine's number one spark plug shall be no further back
than the number one spark plug in the engine originally installed in the
chassis. Where the chassis had more than one type of engine available, the
engine with the number one spark plug that is furthest forward shall be used
for this measurement.
3) In no case may the alternate engine's rear mating surface be further back
than the firewall.
For FWD engines installed in RWD cars, the firewall may be modified to clear the
intake manifold or engines accessories.

b) The engine's crankshaft centerline shall be be located within one inch
horizontally and vertically of the centerline of the originally installed engine
measured relative to the front crossmember. A lower engine location is
permitted for alternate engines which cannot fit under the standard hood
at this height. Such engines can be mounted no lower than is necessary
for major engine components to clear the hood skin by a maximum of one
inch.

I think this language would cover nearly any likely engine swap, and would keep the alternate engine very close to the stock location. A couple of explanations: the language in paragraph a) 2) about two original engines is intended to cover situations such as James Spurling's BMW Z3. In that case, without the "two engines" language, he would theoretically be able to install the alternate six cylinder engine with its number one plug at the location of the number one plug in the four cylinder engine. Also, this would make the FWD-to-RWD swaps comply with the same rules as everyone else, except they have would still get the firewall modification allowance.

I don't know if there needs to be some language on FWD alternate engines, since side-to-side relocations would be difficult and fore-aft limited by the half shafts. The main question is should rotation be permitted to improve halfshaft alignment? I seem to remember that IMSA used to allow engine rotation of up to 10 degrees for this purpose back in the old Proformance series for FWD cars. Maybe that should be allowed?

On the complaint that we don't know what all these measurements are, I think Chip has it right. They don't have to be known at tech, although it would be easy to acquire them for the common cars (Miatas, Rx7s, BMW Z3s, 325s, etc.). If there is a protest for some other car, the information can be found to resolve it.