66 gmc with 307; good choice for towing?

[I1502]

If I can figure out how to post pictures, I'll put some up. Got my 3/4 ton GMC home, found out it was a 66. From the VIN, it was a inline 6, and the empty shift collar meant three-on-the-tree. Hasn't been registered since '85.
-BUT, what it has is a Muncie SM 420 4 speed, and a small block Chevy with "71 307" marked with that yellow paint stick junkyards use. There is a big fan shroud, 7 blade flex fan, and alternator. This is fine for me! The bad news is the 2bbl manifold is empty, and there are rodent nests in the manifold, with some "stuff" down the bores. Haven't tried to turn it over, but the oil looks clean. Suggestions?
Question: Is the longer stroke 307 a good motor for torque/towing, or is a later 350 better? Remember, I'm a Ford guy and really don't know the subtle differences in small blocks.

[stepside454]

for towing a 350 will be better, the 307 was never ment to be much of a performer, it was an early smog motor. However, its easy to make a 307 be a decent performer, & get really good MPG...but if the question is ,what will tow better ? the 350

[bpmcgee]

In the materials that I have regarding my '60 1/2 ton, they list the maximum input torque for the transmissions available that year. My truck has the SM420, and I remember noticing that it's got VERY low maximum torque rating -- in fact it wasn't even OFFERED behind a V8 that year.

This seems strange to me, because it's generally considered a pretty rugged trans. Not sure if this means that the numbers were just wildly conservative or if more modern variants of the SM420 were more rugged. Either way, it's something to consider.

Maybe someone else can shed more light on this topic?

Brian

[60-66 Nut]

I1502, stepside454 is right, the 350 will definatley outpull the 307, but the 307 is a decent old motor too. It was designed using the 283 bore with the 327 stroke, and all 307's were big journal. You can put a 307 crank and rods into a 350 block, use 327 pistons and make a big journal 327, but most people would just as soon build a 350. I just like the 327. But with the gears that 3/4 ton is likley to have, the 307 would probably pull pretty good anyway.
As for the SM420, I'm not sure about the input torque numbers, but I do know they are very tough old transmissions. They were used from 47-69 in pickups and big trucks as well, and I think they were all basically the same through the years, although some of the big truck transmissions may have had bigger input shafts. The SM420 is a very popular trans for the rock crawler crowd, and they put them in Jeeps and everything else with nearly every engine imaginable. I had one in a 63 Chevy pickup with a strong 327 in front of it, and it never even broke a sweat. Like anything else, I'm sure one can be broken, but they are tough.

60-66 Nut

[carryallnut]

For me, the deciding factor would be the weight of what I am towing. I had a 283 in mine and was tired of having to leave a day before my friends and get to the dessert about the same time. Put in a nice 350 and still was waving "bye" going up the the grape vine. Put in a 454 and was all smiles, I even got better mileage. ( I still need a bigger fuel tank) Oh yeah, all these motors were behind the SM 420 and then I put in a SM 465. Why GM never made one of these into a 5 speed is beyond me.

[oldtrucksdave]

I have a smalll block 400 in my 65 sub in front of a 420, no problems. I like the extra cubes of a 400 for towing with a q-jet the mileage is not great but tolerable. 14-15 on the highway 9-10 in town. I have 3.73 gears i think that helps over the old 4.10s. If you can find a 400 I would reccommend it highly. The only problem you might encounter is that it may be hard to keep cool, I adapted a later model fan shroud and a larger fan to mine and it stays cool. In regards to the rodent stuff in the engine i would check it completely, I have a neighbor who had to re-bore a fresh block after a mouse left its mark in the bore, I never knew how corrosive their leftovers were Good luck and have fun. before.

[I1502]

Thanks for the input- I was thinking that a long block 350 will be the cheapest way to go, but if the 307 is not too worn out (or even any good) that it'll do the job. A big block would be too expensive for me! Yea, I also heard that the 420 is a strong tranny. This GMC will be for towing a car trailer and it doesn't have to be pretty, in fact I kinda like the "well-used" look. I'm gonna try to start a new post with pictures, wish me luck! Thanks again!