[altabow]

A quick intro - I live in Calgary, Alberta, have a wife and two kids (18 and 21) and have a total of seven vehicles.
-2012 Ford Escape ( keeps wife happy)
-2002 Pontiac Aztec (daughter - loves it - says it's not ugly)
-1986 Chrysler New Yorker (son - grandma's former baby)
-2001 Pontiac Bonneville (my daily driver)
-1981 GMC 3500 (mine - fully restored and modified)
-1970 GMC 1500 (mine - picked it up with my grandfather when I was 12 - fully restored and modified)
-1975 Datsun 280Z (mine - fully restored and very modified)

Turning a wrench is a hobby. I enjoy body and motor work, hate electrical and admit ignorance on many other things!

So I'm hoping that a few of you out there might be able to offer some advice . My 1970 GMC tranny gave out. 3 speed TH350 that has performed admirably for 110K miles. Rather than having it fixed, I would really like to put in a 4L60 overdrive. I'm aware of the modifications needed, so I know that it's not a drop out/drop in scenario, but I think the upgrade would be worth it. The information available to do this swap leads me to believe that I could do it properly.

A quick look at options here in Calgary yield the following:

-shops don't have a 4L60 to buy without a core and they don't want a TH350
-Gm makes a rebuild with warranty for $1400, but I don't think it has any upgrades or modifications
-There are many places in the US I can buy a tranny. I don't know good from bad.

I'm not looking at a torque monster. I have about 350 HP mated to a 3.08 rear end. The extra torque from the first gear and hopeful mileage increase from the overdrive are all that I am hoping for.

I buy many things from the states and have them shipped to Sweetgrass,MT where I go and pick them up.

Any thoughts on the above would be greatly appreciated.

Thanks - Kevin

[delawarebill]

for info.. a 200R will fit.. just make sure u have to converter for it.. and replace it with the same style tranny u have now.. long or short tail.. the drive shaft will fit too...

[altabow]

Thanks. Yes that is a good option too, but the problems are the same on finding and getting one, which is where I'm hoping to get some advice.

[RWENUTS]

THat tranny swap won't be an upgrade, it'll be a down grade. Sure you get a better first gear but in o/d you'll have a 2.10 rear gear. It'll be constantly shifting down, any gas mileage you have will drop like a rock. You need a 3.73 in the back for it to run right.
Rebuild the T350 and use a 2400 stall convertor for a little more oomph off the line.
JMHO

[altabow]

You bring up an interesting point. The truck is quite light (as far as trucks go) and I know the rpm's would be way down in overdrive - 1900-2000. I thought that this would equate to much better gas mileage, and assumed that for most highway driving it would be able to stay in OD. On the prairies here in Calgary it's pretty flat.

Even if it kicks down a few times, isn't that exactly what I have right now with the TH350? How could that be worse?

[RWENUTS]

It'll be shifting up and down all the time even if you find a flat stretch of highway.
And while it's generally flat around Alberta there's still lots of hills. I know that as I lived in Alberta for 55 years till I retired out here.

[altabow]

Thanks for the advice. I certainly don't want to spend money and time and end up with results that are less than what I have. As I said earlier, I'm not trying to get more performance out of the truck, just faced with having to do something now that the tranny has died. The thought of lowering the rpm's to achieve better gas mileage was very appealling. This fall I moved my son to Grand Prairie from Calgary and the best I could do was 17.3 mpg with a tailwind for a few hours. At 70 mph (110 kph) it is revving 2700.
If I just get the th350 rebuilt, it was mentioned to get a 2400 stall converter. This would only give startup a boost and have no effect on anything else, right?

[RWENUTS]

THe thought behind less rpm equals better mileage doesn't always work. In a computer controlled and fuel injected vehicle it will. You're driving a big barn door down the road. 17 mpg ain't bad. The higher stall is nice around town but builds more heat in your tranny and your mileage may drop further.
Far cheaper to rebuild what you have.