M-20 decoding questions

[69rsss350]

Speedo cable goes thru firewall to the left of the steering column when looking from front of car (towards passenger side of the column) wiring for back up lights and welds for RS headlight brackets appear factory correct. I have been told that by all indications this appears to be a original RS/SS with matching tranny and rear end, just a different engine block (which is not really surprising to me). Now my question is whether I want to drop a big block in it or not. This car is a consistent show winner including Saturday Special Achievment Award Street and Class Runner Up at the Super Chevy Show in Ennis (Dallas) Texas in 2000. I have taken it to two shows since I have owned it and taken home three trophys (2 1st in class and 1 best of show). I'm just thinking that since the engine isn't original anyway...why not go the big block route. I realize that I would probably have to change the tranny and gear ratio but can retain the original parts. Opinions?????