I need an XL Twin spring unit that most closely approximates an old Simmons regular twin mattress that we have - for a family member. As best as I was able to find out (reading the label on the mattress, looking at any tags/paperwork I still have, calling Simmons to ask questions although they no longer have info on this exact model), the old Simmons mattress has 12.75 gauge wire and maybe approximately 600 pocketed coils. It feels like they are definitely edge-to-edge. I am thinking that MAYBE if an ALL Quantum coil L&P unit could be purchased, that might do the trick and would be beyond even the Simmons, as per L&P website coil count. I have already gotten two different spring units from two different MU members that have not worked for this family member (thankfully, one of the units is working for someone else while the other one is destined for trash pickup this week). The two previously purchased units have coils with too large a diameter, it seems, and the complaint is that there is too much sinking in, too soft, not enough support. Frankly I find the old Simmons to be rather hard as a rock, but that is what seems to work. If an entire mattress must be purchased to accomplish this (as opposed to putting components together myself - still have an XLT latex topper, a case too but only 6" so probably too short), then that can be considered IF I can be sure of its composition and firmness.
Hi rvneedssleep!
Welcome to our Mattress Forum! 
It looks like you are well advanced with your DIY/surgery project and collected some good data points with the previous coil purchases.
A little research within the forum and found an old thread where another subscriber deep in the “mattress surgery” on an old Simmons as well. It may be an interesting read for you. The model is not the same, but they believed the coils were a 13.25 gauge (ultimately found they were actually 15.5 gauge)
As you’re well aware, the lower the gauge, the firmer the coil, which is probably why it feels “rather as a rock” to you. You would be hard-pressed to find something with a lower gauge than 12, but of course, the diameter plays a role in this too, the smaller the diameter the firmer the overall feel (all else being the same). Additionally, the number of coil turns also factors in. Generally, you can’t look just the gauge of the coil only, because the number of coils (or the coil density), the height of the coil, the diameter of the coil, the shape of the coil, and the number of turns in each coil, and some of the other coil specs that can differentiate one coil from another can also make a significant difference in the amount of steel in an innerspring and along with the type of coil and how the individual coils are connected together can also make a significant difference in the firmness and response of the innerspring as well.
I am not sure how the Simmons old spring would compare to the L&P Quantum in terms of firmness, except that the number of coils in the L&P in XL Queen is double(1200 vs your 600) (They also don’t have foam encased version in case you want to go that route) But as you know all layers work together and you may be able to fine-tune it to the next level after receiving the unit.
You should be able to work around purchasing an entire mattress especially as you still have quality components around. – I’d make a quick call to Ken at Arizona Premium as he is a TMU expert in mattress surgery and with his 40 years of experience with mattresses and different spring units (old and young)can give you more input. You can see here some of his coil mattress surgery although you are already on the next level with your project. That said…the best way to help your family member would be to involve them in testing and selecting according to needs, preferences, and history with sleeping on other mattresses. This might be the best way to help decipher the ideal coils fit in conjunction with comfort layering. Local testing is usually a good way to go if there is confusion about what they like and need. This may take some of the guesswork out of the equation.
Hoping this gives you some good leads. Let us know how things go with the rebuild.
Phoenix