60’ is pretty soft for a car with big tires that goes 140mph.
Should be close to a tenth better...... low-1.30’s.
Plus, it’s a brick...... which should be negatively impacting the speed(so with better aero it would be even faster...... which would make the 60’ look even worse).

It’s an example of not having the “combo” right.

Converter/cam might be a mismatch....... by the looks of the headers, they probably weren’t used for the dyno test, so that’s an unknown variable.
Footbrake or transbrake?
Glide?

Since the speed is way down....... if the alleged dyno numbers are correct....... there could be some other issues in the car costing some power.

I have seen this type of situation play out before.
Car owner spends thousands on car upgrades(converter, fuel system, carb, etc) to no avail, then has motor dyno tested only to discover the car is going as fast as the hp it produces will go(motor doesn’t have what was expected).

I like to use 90% of the STP dyno power as a point of reference for how quick/fast the car “should” be able to run. Then add in any extra ET correction for tracks at higher elevations(over 1000’ or so).

So, working that backwards, 3000lbs, 140mph...... shows 630hp....... which is 90% of 700hp.

Frankly, with what’s available for BBC parts these days...... if one set out to build a “bracket race” 540....... it would be fairly hard to end up with only 700hp.

If nothing obvious in the car reveals itself, I’d put an engine dyno test pretty high on the troubleshooting list.

If the motor tested at 800hp, then the 90% number I’d be looking at is 720hp....... which “should” be going 146@3000lbs.

Sometimes the simple things can be revealing....... do a compression test and check the lash.