7m is going to be too long for the RA21.
But if you want a straight 6 in your RA21, the 1GGTE is almost a bolt in to replace the 18RC.
otherwise, if you want to stick with North American easy to find parts, then the sr20det or even the 4g63t rwd options would be nice...
if you want to stick with toyota, then the 20r/22r or the 2rz/3rz are the best ones. the 2rz and 3rz respond to boosting very well.