It looks as if you have already made your decision but allow me to add my 2 cents. If you are going to keep your car I would do the 4-speed and don't worry about the naysayers as you are building it for YOU. I also agree with a previous poster that a well-executed 4-speed SS454 would probably carry a premium price over a stock car. After all, GM made 4-speed Montes from the factory in 350 and 402 configurations so go for it. I am a Corvette guy and even on the Board of Directors of an Ohio NCRS chapter and know all about numbers-matching but all these guys seem to do is look at their cars and treat them as museum pieces. I however, like to DRIVE my cars and am a resto-mod type of person. I try to tastefully modify my cars to improve their driveability but not make things obvious so that the cars maintain their original personality. I did a frame-off restoration on my '65 Corvette coupe to which I added FAST throttle-body electronic fuel injection to the roller-cam motor, a Tremec TKO 5-speed and rack & pinion steering. It is a BLAST to drive and the average enthusiast doesn't have a clue as to what i have done!
To finish this opinion I will mention that I also own a '70 SS454 Monte that I bought several years ago that had already been converted to a 4-speed, probably decades ago from the looks of it and using all factory parts. The car is fun to drive and drive it I do. In order to improve it even more I just finished up a year-long project to further improve it by building a roller-cam internally-balanced 454 topped by an MSD throttle-body electronic fuel injection and backed by a Tremece TKO 600 5-speed. It looks stock but the driveability will be improved considerably. So, are you restoring a museum piece OR do you want to have fun banging gears? Again, just my 2 cents.