I know you said your engine is a 1958 is that the same year as the driveline. What I am getting at if you have an open driveline what I would recommend is doing a T-5 conversion instead of the SM420 4spd. tranny. There is very little modification needed and if you get a T-5 from a early s-10 you still with have the mechanical gear for the speedometer and the shifter location will be close to the original location unlike if the T-5 is from a Camaro where the shifter on the tail housing is further back.
I purchased a T-5 and it bolts directly to my clutch housing and I just need to purchase a spacer because the input shaft is just a little long. Other than drilling out the 4 mounting bolts its a good fit on my 235 motor.
Here is the way it was explained to me about changing the rear gears vrs. tranny with overdrive. If you have the 3.90:1 gears there is only one gear set made for it. A 3.38:1. Plus you have pretty much put new seals in the rear end and while your in there just better replace the bearings etc. so now you have the expense of labor , gear set, and rear axle seals. Now if you just replace the tranny you can save some money by just replacing it.
Now I talked to the rear end shop in town and they suggested in replacing the whole rear end with a newer style so if I ever decided to swap out the internals it can be done easier than with the 3.90:1 Eaton HO33 style I have.
There is many threads here you can read about the t-5 swap. plus there is a guy here that has a lot of experience that might chime in named lugnutz. here is his page
http://lugnutz65chevystepside.weebly.com/ Here is what I did on mine. I am running the 225/75R-15 tires which has an overall diameter of 28.29" and fits the original 5" rim. (Sidewall Height of 6.64, Section Width of 8.86, Tire Circumference of 88.87, and Revolutions per Mile of 735.24). I have the Eaton HO33 3.90:1 original rear end with an open drive line. Borg Warner T5 5 Speed O.D. Transmission Model #1352-145 from an 1985 or 1986 S10 Engine 2.5 L4 or 2.8 V6. (1st gear 3.76 2nd gear 2.18 3rd gear 1.42 4th gear 1.00 5th gear 0.72). Mated with my rebuilt 1956 235 engine.
When I run the RPM numbers here is what I get.
70 MPH = 2335 RPM
65 MPH = 2168 RPM
60 MPH = 2001 RPM
55 MPH = 1834 RPM
Engine RPM Caculator If you are planning to go 70 MPH or greater you may want to consider in upgrading the suspension or braking. Just a thought.
You also need to think about how you are going to use it. Daily driver, towing, going fast from stop light to stop light etc. My set up is for driving around town hitting the freeway with lower RPM's now and then with out making people mad because I am going slow on the freeway.
Just my .02 cents good luck and keep us posted.
