Theres been an original Doug Nash 4+1 5 speed,(non OD),in my Monte ever since I bought it 17 years ago.
I currently have a Ford 9 inch with 3.0:1 rear gears.With the 3.28 first gear it is equivalent to having a 4.10:1 rear gear if compared to a 4 speed Muncie,but with the 1.0:1 fifth gear it is equal to having a .64 OD.
I would not recommend anything lower than a 3.08:1 rear gear,a previous owner told me that at one point the Monte had a 3.25:1 rear gear and first gear was useless and he just used 2nd gear starting off.
With 425 HP you should be safe from breaking anything unless you launch it at high RPM.
They can be blueprinted and made to work better,here are two videos that I had my transmission guy watch last trime he rebuilt it;
The gentleman that made these videos,Paul Cangialosi,really knows his way around these transmission's.
I talked to him and bought the better synchro rings and spring clips and also switched to Royal Purple 75W-90 Gear Max synthetic oil that he recommends.
I also had 1st - 4th gears,the main shaft and input shaft cryogenically treated for more strength.
It shifts real nice now,the best it has ever worked.
IMO,selecting the correct rear gear ratio is the most important factor with these transmissions.
Ideally,you want a SLR,(starting line ratio),between 9 and 11.This is found by multyplying first gear by the rear gear ratio.Mine works out to 9.84,(3.28 X 3.00),which I find works very well.Here is a chart from Wallace Racing;
http://www.wallaceracing.com/calc-slr.php
Hope this helps.
Guy