I orderd a set of conrod bearings to suit Magna 6G74 received RB 1180 .
These bearing shells measure approx 15.1 mm wide where as the ones of the car measure
20.7 wide. This car has had a late model new short motor fitted. Has anyone struck this before.

All I can tell you is that this is what ACL quotes as the specs.

Conrod Bearing Set 6B1180
Oversize STD, .25, .50, .75, 1.00
Brand ACL Duraglide
Manufacturer ACL
Conrod Bearing (c) 1180
Half Type Identical Halves
OE Part Number 1 MD311930
Manufacturer ACL
Min Std Shaft Size 2.1646in / 54.982mm
Max Std Shaft Size 2.1654in / 55mm
Min Std Tunnel Size 2.2835in / 58mm
Max Std Tunnel Size 2.2842in / 58.018mm
Max Wall At Crown 0.0589in / 1.495mm
Max Overall Length 0.6102in / 15.5mm
Min Vertical Oil Clearance 0in / 0mm
Max Vertical Oil Clearance 0in / 0mm
Material F780

The OEM Numbers for the 6G74 & the (3.8L)6G75 are the same . They are MD311928,MD311929,MD311930,MD311931,MD311932 and this would allow for variation in journal diameter sizes.
Note One of these numbers matches the OEM number ACL quotes.

I wonder where your block / crank came from or whether it was modified?