Don't buy wheels on spec. They are more likely not to fit. Lots of people have had this problem. Worse is if they almost fit and rip the front fenders up on the first sharp corner with a bump.
Info from Tim Engel:
-----------------
13 x 4.5", +1.062" Offset, +27.0mm ET - Europa Steel Wheels
13 x 5.5", +0.637" Offset, +1.62mm ET - Europa Alloy Wheels
13 x 4.5", +0.812" Offset, +20.6mm ET - Spitfire Steel Wheels
The Twin Cam's Brand Lotus alloy wheels are not only 1" wider than the standard steel wheels, the smaller positive offset moves the wheels outward by 1.062 - 0.637 = 0.425".
Straying a little off-topic, but the Triumph Spitfire steel wheels some Lotus owners use also have less positive offset, which moves the wheels 0.25" outward compared to the Lotus steel wheels.
-----------------