Yeah, the exhaust is very much a "How can I get it driveable, quickly" setup. Will 2.5" be enough or do you think I'll need 3"?

 

 

I would figure out why it won't rev above 5,000rpm and fix it but don't concentrate on making power there.
For towing you want maximum torque in the lower range 2,000-5,000 where it's going to used most of the time.
If 1:1 is your final drive ratio then use the calculator posted about to find your target speed and then figure out what changes you need to make to get your RPMs to fall into the range you want to run the engine at. My truck has overdrive and with 3.73 gears it turns 2,200 rpms at 70mph (112km) With a 4x4 going up or down in tire size is sometimes cheaper than changing gear ratio if it's close already. I used to drive a 1968 Chevy truck with a 396 big block and 5.13 gears, It would pull a house but with a 1:1 final drive ratio it topped out at around 50mph.

 

It will rev beyond 5000 RPM, it's just flat.
What are your towing weight limits in the states out of interest?

 

simply a fueling and timing adjustment. Your power enrichment is probably set from factory to engage at 5000 rpm which is when the overly rich stock commanded air/fuel ratio comes into play and does nobody any favors. Timing is usually in the low teens at higher rpms in the higher load cells so that's also part of your 5000+ rpm weakness.

 

 

Yeah - Legal limits, I've seen 4x4s pull road trains and Jumbo jets..