Kamzeride,
The pic below shows you will be fine with your proposed approach. This is a Z420 v2 with a E5-1660 v2 under an unmodified Z440 "Performance" heatsink/fan. These currently go for roughly $21.00 USD with shipping from eBay, and search for749554-001 there. This Z420 v2 also has the optional Z420 airflow baffle, and the stock Z420 rear case fan, and the stock Z420 rear memory bank fan. All fit in nicely, as does the front bottom optional case fan you know about.
The red cable you see is to add eSATA II out onto the backplane. There are only 2 SATA III ports in the Z420/Z620... I reserve one for the boot/apps SSD and the second for a documents SSD (not yet installed). I could get eSATA III via one of those two if I needed. The HP "2x2" Texas Instruments chipset USB3 PCIe card there beneath the video card is present to feed a front HP USB3 SD-card reader/writer via its USB3 "motherboard-type" header. That card also adds 2 more full-power USB3 ports on the backplane. This was a build for my better half... which has since morphed into a Z620 v2 single processor project using all the same parts plus one of the Z620 memory fan "saddles" that perfectly fits over that Z440 heatsink in a Z620 single processor build.
I used to think that I needed to swap in to the Z440 heatsink a Performance fan from a Z420/Z620 heatsink, but then realized that you don't need to do that at all to use this double capacity Z440 heatsink/fan in the Z420/Z620 (these are for a single processor build). The Z440 heatsink's fan plug has 6 holes, but the last two are just a ground-ground jumper the Z440 wants to see. The Z420/Z620 do not need that extra hole so just hang that extra 6th hole off the side of the Z420/Z620 motherboard's 5-pin header, out in space.
The pinout for a v3 or v4 processor does not match the socket for a v1 or v2 processor, but the heatsink mounting holes and spacings are all cross compatible between the ZX20 and ZX40 sockets. These larger Performance Z440 heatsinks have almost exactly 2x the cooling fin area and 3 instead of 2 heat tubes when compared to the Performance heatsink for a Z420.
I've been dinking with Throttlestop a bit, but need to create some time to use Brian's great guidance on getting XTU to run properly, but under W10. I am forced to use W10 over W7 now for security issues at work. You may have seen my post here on turning off "Fast Startup" in W10 to help XTU work properly... in another post. That has helped us save use of literally several $100k USD of medical monitors that need brightness/DICOM control software to work under W10, using software built for W7. I got a free latte for that discovery. By turning off Fast Startup under W10 all work perfectly now under their W7-written control software. And, XTU does too per the post you can find here from the recent past.
Here is your pic... Note the big Z440 heatsink/fan in place. You will not be able to see it until a moderator releases it:
