My Series 3

Have I made a mistake?
I’ve recently bought a 1979 109, I believe originally it had a straight 6 petrol and now has a 19J installed with overdrive.
Having finally been out for a drive, disappointed, is an understatement. I knew it would not be a shogun, which it is replacing, but 45mph flat out is very very poor.
If you push up to 47~ there is a HUGE flat spot no power nothing the engine dies almost cuts out.
Slow down, shift into 3rd/2nd back off the throttle then re apply it picks back up.
Could it be the long induction pipe from the original oil bath air cleaner?
The poor top speed, are the gear ratios/diffs out of sync with the diesel?
I plan to tow my large caravan with this 109 but I’m thinking it will not be capable.
Anybody have any suggestions?
Thank you in advance

Any photos of the install? The 6 cylinder has the gearbox further back in the chassis. And it's a different bellhousing, I think.

And any more details on the install...

When was the engine last serviced? What fuel pump does it have? Fuel filter?

I've a tweaked TDI in mine and at 60 mph towing is loud, but not a massive strain on the vehicle.

But, yes, not a shogun.

Trying to attract pictures.
Currently the only photos of the engine bay, one other thing the exhaust system is a Steve Parker system.

Assuming the pistons aren't knackered that should do a hell of a lot more than 40+ mph.

So, ditch the oil bath, which could lead to a run away engine. A TDI filter is better, though looks odd.

Change the fuel filters, check the brakes aren't dragging and that the throttle pedal moves through the full range. Failing that, timing belt, fuel pick up pipe, fuel lift pump.