The Evo will bolt right in, if the frame has been clearanced at upper seat post ( if aftermarket frame, most likely done already). If the Twin Cam has the 4 bolt mounting flange it will require an adapter to Bolt in. Unless the T-C has gear drive cams, I'd go with the Evo. There are tons of parts available for not much $$ for both engines. The T-C will vibrate in a solid mount frame, more than an Evo, unless it has been balanced much better than standard factory issue.