Sounds like a great engine for towing. TJ, have you considering a BW T-19 truck 4 speed? Although still a heavy cast iron trans, it does have a syncronized low gear, and has what I consider, much more usable 1 rst gear ratios. I believe the NP435 uses a 6.8ish 1rst gear, the T19 has a 4.0ish 1rst for diesel applications, and a 5.0ish 1rst gear when used with a 460 engine. However, BW also made a T-18, which was geared very similar to the NP435, and does not have a syncronized 1rst gear. Both the T-18 and T-19 look quite similar to each other, to tell them apart, a T-18 has a single PTO cover, on the drivers side, while the T-19 has 2 PTO covers, one on either side.For 26 years, I hauled my drag car with a 74 F350 DRW ramp truck/car hauler, normally weighed about 10,500 lbs, powered by a mild low compression 390 4 barrel engine from a 75 or 76 F250. Back in 1990, I just honed the bores, installed new rings on the original "410" cast piston, usual new bearings, valve job, timing chain etc. The only changes I made were a set of full length headers (Hedmans from 1990, FlowTechs about 10 years ago), a locally ground "RV" type hydraulic cam, and a C6 "S" cast iron 4 barrel intake mainold with a 450 cfm Holley carb. I also used the electronic "Duraspark" ignition from the donor truck. Mt truck had a NP435 trans, and 4.10 Dana 60 with 7.50R-16 Michelins, that were almost 32" tall. At 60 MPH, the RPMs were about 2700 RPM, the more common 235/85R16 truck tires are a bit shorter, so the RPM would be somewhat higher. I rarely used 1rst gear, unless starting off on a steep hill while loaded, but even still, the super low 1rst gear ratio meant I seldom got more than 10-15 MPH in first gear, I think the 5.0 low T-19 would be much more usable, plus being syncroed, much nicer to use. I realize that you have probably twice the power my 390 had, so a rear end ratio change may be OK, I never considered than myself.
As for a O/D 5 speed, my current tow setup is a 99 F350 DRW Super Cab 2wd, with a 6.8L V-10 and a ZF 5 speed trans, and Dana 80 with 3.73 gears. Tires are 235/85R16s. Not sure what the 1rst gear ratio in my ZF 5 speed is, but it is more usable that the NP435 in the old truck, plus is syncronized,but with 3.73 gearing, I normally tow on the freeway in 4th, only use 5th on long flat stretches, or at higher speeds. I tow a 24 foot Wells Cargo enclosed trailer, my truck alone is just under 7,000 lbs with me in it (no camper ,canopy, or any load in the box normally),and my loaded trailer is normally 7500-8000 pounds, so I am considerably heavier than the old ramp truck was. In O/D 5th, at 60 mph, the engine is about 2100-2200 RPM, in 4th its 2600 RPM, which is not too bad. In O/D 5th, there is virtually no throttle response, any hill requires dropping to 4th. Again, maybe your much stronger engine wouldn`t mind the O/D, or a taller gear ratio. I bought a used 5.0 low T-19 a few years back, to put in my 74 truck, but sold the truck before that happened. I understand the T-19 is a straight bolt it swap, my T-19 was from a 1984 F250 2WD, with a 460 engine.
