As I recall, you have a nicely warmed 350 with 3.42 gears in your truck. I don’t know what size tires your running, but for purposes of discussion, let’s use 275/60 15 (I’m running these on my ’71 pickup).

With the 3.42s and a stock TH350, your truck should be pretty good fuel wise, i.e., at 6000 RPM the “roughly calculated speeds” are 58MPH in low, 96MPH in second and 145MPH in third (approximately 24.33MPH p/1000 rpm)! This doesn’t take drag coefficients into play – but even still, that’ some fairly high gearing. I think your poor mileage may be due to too high a stall speed or slippage from a tired converter. If you have a 3,000RPM stall you’d need to be going almost 73MPH in third gear before the converter actually stops slipping so it may be that you’re slipping at freeway speeds.

With a 2000RPM converter – you’d be golden on the freeway as the converter would be fully engaged at 49MPH. This is why the newer cars all have multiple speed transmissions with overdrive to allow a very low stall speed (some as low as 1000-1200!) to allow say a Ford Expedition to cruse at 70MPH and only be turning 1800RPM.

Now – to your question

700R4 with 3.42:

1st gear, 3.06+3.42= 6.48
2nd gear, 1.63+3.42= 5.05
3rd gear, 1.00+3.42= 4.42
4th gear, 0.70+3.42= 4.12

Your current TH350:
1st gear, 2.74+3.43= 6.16
2nd gear, 1.57+3.42= 4.99
3rd gear, 1.00+3.42= 4.42

The 700R4 is going to give you more snap through the gears and you will have a bit of overdrive in 4th gear for cruising the freeway.

The TH350 is a stronger transmission from the factory but a 700R4 properly built will be more than adequate for your truck.

I’ve not personally used them – but I hear nothing but good about Bowtie Overdrives in Hesperia – probably worth the drive (yeah I know – another tank of gas) from your place to talk with them and get their inside story. Their prices are very reasonable and they just do GM transmissions.

http://www.bowtieoverdrives.com/index.shtml

Let us know what you decide.

Merry Christmas,
Glenn