Its Ray, not Paul, Roger!!!! I never had an issue with airflow using a standard bonnet and the twin 1.5" SUs (which I also still have) as I did the same grille drop with the tattiest Wolseley Hornet you have ever seen, (same engine, but before a full, rebuild) which I sprinted and hillclimbed in the UK, with a fair bit of success, in the under 1300cc road-going modified class... That shell got left behind in the UK, but all the mechanical bits came with me.
With the Wolseley, I just cut the bottom off the grille, but with the Riley, I cut back the front valance and parked the oil cooler there and retained the full length grille, which looks better IMHO.
I had a fibreglass bonnet for this car and dropped the grille as my pet hate is bonnets that lift with the grille, usually making access difficult, not to mention smacking your head on it! When I look at my drawings of what the Broadspeed Elf was supposed to look like, I thought it would look pretty good, as the rear overhang was better looking than the standard Broadspeed with the GT rear end on a standard Mini base.
It was always going to be 'SMILEY' the Riley so I bought the personalised plate as soon as they came out. That plate is now on the Marcos, which has been off the road since 2006. The car was on the road legally very briefly in the early 1980's, but sadly, I never got the chance to see what it could really do, so the engine has only done about 200km running in since the build by a specialist in the UK who competed with a Turner (BMC A Series engine).
I have done the same with the Magnette project. The grille is now fixed - though easily detachable.