I've been keeping my speeds at 55 whenever possible out of fear of overworking the 20r under the hood.  Every so often due to traffic demands I'll take it to 60-65 just to keep pace.  But I'm not sure if this will just wear down the motor prematurely.  I've noticed other guys on this forum still keeping the original 4-speed, so I'm not sure if I'm unnecessarily worrying myself about overworking my engine and tranny running 60-65mph.

Would it be difficult to find a Toyota 5-speed that would be a simple bolt and go swap?  

there were 5 speeds for the 20r, and i think the later 5 speeds will fit as well, check out http://www.toyotaminis.com, use my username as a referral (its the same as this site)