Chassis Dyno results to estimate gross engine rating ?

Dyno rear wheel HP/TQ how to estimate gross engine HP/TQ ?
Chassis Dyno results to the rear wheels: 330 HP to the rear wheels peaking at 4800 to 5000 rpm , and 405 ft/lbs torque peaking 3600-3700 rpm .
I have a 70 cuda, mostly factory stock equipped B-block engine (stroked to 431 ci) MR 6 PK cam, stock heads with power steering & all stock factory accessories driven, A-833 4 spd trans , & 8¾ sure-grip rear with 3.23 gears.
Dyno operator suggested a figure of approx 18% losses to calculated 1970 type gross engine ratings, so therefore that works out to approx 400 HP & 490 ft/lbs if comparing to 1970 engine Gross ratings . Does this sound reasonable ?, Has anyone had a chassis dyno after a engine dyno with a similar combo – PS, acc driven, 4 spd, & 8¾ rear , to know what HP or TQ losses are typical to my driveline . thanks Doug , dougandcindy@sympatico.ca ( moparts user SCHU ).

18% sounds like a decent number to use for an old-school drivetrain. Newer cars are more efficient but the stuff designed back in the 60's didn't have access to the current technology.

I used 15% on a 383/450" turning a WP and alt, a 4-speed, and Dana w/3.73s to arrive at 560 HP and 585 TQ with more tuning in it.

OK so it sounds like my driveline combo takes takes approx 70 HP & 85 ft/lbs to drive,
( 833 4spd & 8 3/4" sure grip, with all factory engine driven acc incl PS).
thanks for your input, SCHU .

Mufflers and what kind of set up equalizer pipe (none) or X pipe out the back past the bumper??