At some point it my 1978 Silver Anniversary's life it was converted from an automatic to a 4spd, I don't care because I just want to have a nice driver not real worried about the numbers matching and I enjoy driving a standard but my rpm's are high in 4th at highway speeds  60mph at about 3500 rpm. I have not checked the rear end ratio yet but would leaving the ratio for the automatic cause this problem.  Any other suggestions!

Thanks

I think your gears are non OE. 3500 RPM at 60 MPH would suggest 4.10 gears. My 4 spd with 3.36 gears tachs around 2600 @ 60 MPH. My 57 BelAir with 4 spd tachs 3100 RPM with 3.73 gears at 60 MPH. Similar size tires.
Dave

The car prolly has a 3.55 rear ratio if it had the automatic. There was a 3.70 ratio available, but only for the 4 speed cars. The change in trans from auto to 4 speed does NOT change anything as far as rpms x mph on the road, since both 4 speed and automatic is the same high gear ratio...1:1, or direct.
You can look at the numbers on the bottom of the diff to see what it originally had, but that's no guarantee that the gears have not been changed somewhere down the line.
I'd guess the car still has the original 3.55 ratio, but who knows what has been done before you got it.

Here's a good engine RPM calculator you can plug your numbers into. I send this to my customers when they are trying to pic a gear for our rear ends.  Click HERE...

If I recall the stock Turbo 350/400 torque converter runs at about 85-90% efficiency which relates to something like a 200 RPM higher rate with the same rear end ratio (compared to a 4spd) due to slippage. The above calculator compensates for this. You can jack the rear end of the car up, have someone turn your rear wheel exactly 1 turn by hand while you count the times your drive shaft turns over. You want to mark it with a fixed ref point off to the side of the drive shaft so when your down to that .36 .55 .70 .10 of your last turn you can figure your gear. Its doable....for example the dif between .36 and .55 is 68 degrees of drive shaft turn. You can visually see this with masking tape and sharpie marks. Hope that makes sense!
Both tires will need to be off the ground with a posi....turn the wheel forward.

I can't speak for the autos, but there is a reason that for many years the 3.36 ratio came standard with the L-48 4 speed cars.  It is a good all around ratio that does not have you screaming along just going 60, but still decent enough off the line.  I wouldn't dream of changing it considering I've rebuilt for a pretty good bump in power (alleviating the need for lower gears), I plan to keep my original borg-warner (vs putting a 5 speed OD in for a lot of money) and I use this car as a cruiser vs a drag racer.   

What's also helpful, is knowing what 4-speed trans you have.  Hopefully, you have the Muncie wide ratio trans with the lower 1st. gear (2.52).  If you have the close ratio (2.20 first) then you might want to consider nothing numerically less than 3.55 ratio rear gears.  

Definitely 3:55.i run them with my T 400 auto. Rpm matches.

You have a 4.56 rear end.

http://www.prosystemsracing.com/calculate.html

I think we should place some bets  . We be all over the board....