Unfortunately personal, health and time issues prevented me from progressing any further with this thread. Well, its nice and warm outside, the season has 'kicked-off' and I spent some time with the idle bleed jets earlier today.
Recapping, my problem was an off idle 'hole' at approximately 1100 rpm. It showed itself when cruising in traffic and gently accelerating from 1000 rpm or so.
Thanks to all of you who contributed especially Motorhead who kinda put me on the right track.
I did swap the original no. 33 primary squirter for the no. 35 secondary one - this didn't seem to help.
The Proform 950 dp wasn't new when I bought it (supposedly only used for a dyno test). When I checked the manufacturers specification I found the following Hi-Speed bleeds were no. 32 all round (should be no. 31) Idle Bleeds were no. 71 all round (should be 73)
So just concentrating on the immediate low speed issue and the idle bleeds it seems that changing to smaller jets will richen-up the idle mixture and take the transition point from off-idle to main jet operation a little higher up the rev band. I swapped-in no. 69 jets and tested car - perhaps a little better but still not right. I then swapped-in no. 65 idle jets - and took the car out for a test. Now this DID show a definate inprovement - the 'hole' around 1100 rpm was still there but nowhere near as noticeable the REAL surprise was the improvement in transition between, say 1200 and 1400 rpm - it was far smoother than before and now in traffic I can actually hold 30 mph at 1100 rpm in 4th gear and even accellerate (carefully) - I have a full set of Hi-Speed and Idle-Bleed jets on loan so I will see how it goes and swap-in a smaller set of jets if required. As for the Hi-Speed bleeds, I will leave well enough along for the time being - I may swap-in a larger still primary squirter just for good measure - thanks guys once again for your input.
BTW did my first 1/4 miler at local WW2 airstrip - poor surface and traction (concrete surface) - best was 13.27 @101 mph.

 

I would say that allot of your problem is low vacuum at lower rpm. You are expecting to much. My present motor idles at @ 1000 - 1100 rpm hot. I hooked up a vauum gauge and turned the four corner idle screws down to get the highest vacuum and did a test drive and it was horrible. I had to back them back out to get the best driving back. Adjusting the air bleeds allowed me to run the smallest jets ever. But the down side is that it became very sensitive to idle screw adjustment.

I've been have lots of fun just out driving since I've started it's conversion back to a street machine.

 

Checking the vacumn out was something I was going to do yesterday - my son has seemingly 'borrowed' my gauge

 

roscobbc
which spark plugs are you using?
my pump gas L88 behaves like your ( but the hole is bigger: 1200rpm idle, hole up till 1800rpm. that's the cam...)
i had an improvement in low speed operation going from heat range 3 sparks to 4 :autolite 3924 gapped at .040.
I also closed a little the primary butterfly to reduce the idle and seemed to help ( i think a better transition from idle to main)
I already played with squirters and pump cam and found that squirters size do little ( i use 35) instead the cam makes a lot. with my holley I use the pink (?) cam but used in a somewhat "odd" overadvanced position that works fine

 

NGK BCP7ES and yes pink cam as yours - MSD seems to fire through any rich mixture issues?

 

So I now have the smaller no.65 idle bleeds and I finally bumped-up the primary squirters yesterday from no. 35 to no. 40 but tube type. I wasn't expecting a change with this but change is exactly what I got! - it was sufficient to overcome the previously reported 'hole' at around 1100 rpm, I can actually drive in 4th at 30mph (1150 rpm or so) and the hole has gone. I'm not saying it will pull happily at this speed 'cos it won't - but it will with gentle throttle use maintain the speed - it is much nicer generally in town driving too. All I have now is a slight hunting effect when slowing down at these sort of revs - you can feel it as slight drive line shunting - I'm going to guess that it might be a side effect from a potentially rich idle mixture (caused by the decrease in idle bleed size) - I'm not too bothered about it.
So thanks guys for all the advise, support and tips.

 

rosco
no.40 squirters are really big. on my 440ci i went down to 31 from the previous 35.
i look forward to address a bit the low speed operation by using an annular boosters holley 850 in place of my downleg boosters 800

what's the timing of your engine? i run a lot of advance ( static and fast centrifugal , all in at 1700 to help idle and low speed)

 

My old and basically stock L36 had initial of 8 degrees,total of 36 degrees and 'all in' at 2000 rpm. The 489 has been set with 22 degrees initial, 38 degrees total and 'all in' at 3000 rpm.