ORIGINAL: proanti1
Most .21's are big blocks, which require you to drill up the chassis and use different mounts. You could throw an OS .18tz in it, which would be more than enough power.
Couldn't you use a 21TM? That is the only big block engine I know of with a smallblock mount. Has the same footprint as an .18
AFAIK. Its probably too much power, but wheelies can kinda be fun... for awhile.