Trans swap ?'s NP435 toTH350

[Beejer]

I have a 1969 GMC 1/2 ton long bed. It has a 350 engine, 4 speed manaul transmission NEW PROCESS NP435 a popular low gear ratio 4x4 transmission. GM, Ford, Dodge, and International all used this transmission in their earlier years. Although GM rarely used this trans, I happen to be one of them. I'm thinking of swapping the NP435 for a TH350 transmission. My question is what kind of process am I looking at? Besides the new bellhousing and transmission what else needs to be altered? Any help would be great!

Thanks,
Beej

[Wrenchbender Ret]

You will need to cut out the cross member under the bell housing & put one under the rear of the trans. You will need a strg. column for an auto matic or a floor shifter. That would be the major modifications. What rear end ratio do you have? Nowwould be a good time to upgrade to an over drive if you need a more highway friendly gear ratio.
GMC used a lot of the New Process trans. The early ones back then were not very good. They would break syncro's & had a sloppy gearshift feel.

[Beejer]

So I cut the front cross member to fit the new trans, and add another for support to the rear end of the trans? What's the differance between a short or long tail TH350, other than it's length? Does one perform better than the other? I would prefer a floor shifter, because I plan on building a custom center console. I'm not sure of the gear ratio. It's a stock 12 bolt rear end, 6 lug, that's about all I know. I need to change the differential cover gasket soon, so I'll get back to you on that.

Thanks,
Beej

[68C15]

my 68 GMC led a pampered life with the NP435 and it worked great when I did the same swap. you need to completely remove the bell housing crossmember and install one on the rear of the TH350. I fab'd one out of 2" square tube and a chunck of 2 1/2" angle.

when mine was still leafs in the back it had a Dana 44 with 3.42 gears

[rsavage]

I just changed out my SM465 to a 200 4r. I bought an original auto crossmember from one of the members. I ground off and drilled out the rivets so that the bottom piece could be put in separate from the two ears for the top of the frame. I bolted it back together with grade 8 3/8" bolts. The front bellhousing crossmember has to come out. With a body on the frame, air chiseling the rivets and again, taking it out in 3 pieces worked well.
You will need to have your driveshaft shortened and if the 435 has the big ujoint on a yoke that is bolted to the transmission, you will have to use a transition joint. You will need a yoke for the 350 - I think that is a 27 spline too like the 200 4r. You will need a block mounted starter if you currently have the bell housing mounted starter, and you will need the appropriate flexplate and bolts for the THM 350. You will need the cooling lines and an auto radiator or a remote trans cooler. You will also need a brake pedal setup to replace your clutch/brake pedal set up currently in your truck - again available from members on the parts board. Trans mount, filter, gasket, and fluid all available at your local parts store and a tranny dust cover if it doesn't come with one.

[Beejer]

Thanks 68 nice ride! So I need only need to fab one cross member for the back of the new trans correct? Is your truck an automatic now? My truck has coil/spring rear suspension. The rear end is a 12 bolt, with a 8-7/8in ring gear, any way of figuring the ratio by that?

Thanks,
Beej

[68C15]

rsavage is right except for the clutch-brake pedal deal. you can slide the clutch pedal shaft out of the brake pedal hanger after you remove the right side arm from the shaft.

[Beejer]

You've both been very helpful! It's sounds reasonable to do, so I will be on a parts hunt. I would like to find a used TH350, does the year matter, do I need an older trans also? I found a conversion cross member for $89 locally, so I bought it. I will let you guys know as it comes along.

Thanks,
Beej