Sell the twin 1 1/4 on ebay, they will fetch $300 if in bling bling condition. Call around and you will be able to find a redline medium length manifold for $180 approx. This plus the 45 and a lynx ramflo 445S P5 Offset filter (with foam and oil) will just fit without knocking the firewall. My suggestion is to knock it in alittle to be on the safe side. All in all don't let it sit on the firewall.
Sell the 40 with manifold and get $300 or so for it. Then you'll have spare change
But I'd definately go the 45, esp if you want to mod the engine later on (if it aint already modded).
Also I linkages, jets, chokes ... if it doesn't already have linkages then you're looking at the bad side of $100 for a set, so if the 40 has linkages, and the 45 doesn't... you get the idea... Not sure if jets, and chokes are interchangeable but you are looking at $50 -$100 depending on what you need to change there too (example: air correction jets, $18 for one and you need 2!). Just be mindful that just cause you have the carb, that's not all you need, as chances are it's missing this missing that.
There are pics of my setup with 45 DHLA and a redline medium length manifold (I think) here and as you can see, the clearance is tight to say the least!!!
http://ausmini.tziworld.com/viewtopic.p ... highlight=