Several issues at play here.
In my opinion, getting rid of the rubber ISO-clamp biscuits is one of the best bang-per buck mods you can make, for it will improve ride many times over.
When new, the rubber might isolate the road to car, just fine, but after a few short years – not so much (and it compounds ride harshness, wheel hop and other issues when the rubber breaks down).
Chrysler has not made new rubber ISO biscuits for a couple decades. Replacing the rubber biscuits with urethane (or polyurethane) is one option. This allows you to use the original hardware. Several members here have gone this way with great success – but I do not prefer it. The lower U-shaped bracket is weak at the bolt holes and I have seen several crack/fail over the years. If this bracket breaks, your car will need a tow truck to get anywhere – for the leaf spring will be dragging ground and the rear wheel will be contacting the inner fender well.
The area in blue circle is where it is weak at (which you can see is bent, in the picture).
Also, if upgrading to 5-leaf springs, the lower U-shaped channel is different (1-leaf taller - see red line) – so one more part to have to locate.
To get rid of the ISO-clamp altogether, you have several options:
One is to use a shock plate made from ‘60ish to mid ’70’s from any A, B, C and E-body with the following exceptions: ’70 E-body plates are made different (1 year only plate) & any 7¼” differential (which has a 2½” tube vs. 3” tube for all other Chrysler differentials). Chrysler started to use the ISO-clamp in the early ‘70’s on a few cars and on most all cars by the mid ‘70’s. The FMJ ISO-clamp is different than the other body style clamp systems are.
If you use an older shock plates – also locate rear shocks for any ’66-79 B-body (which is a more common part than FMJ rear shocks, are).
There are other companies (than Firm Feel) who also makes shock plates and I have seen some new plates around the $60-dollar range. I have purchased used shock plates in that same dollar figure – but most people are asking for about $100 for a pair.
There is nothing wrong with making your own plates – if you have tools and patience to do so. Personally, I’d rather just get a set, ready to go.
The Firm Feel set allows you to use the existing top ISO-plate (to use original shocks) and a different lower plate (not an older style shock plate), so you can toss the rubber and that style works fine, as well.
https://www.ebay.com/itm/Mopar-Dodge-Plymouth-8-3-4-Dana-Shock-Plates-MMR-0178/183559364777?hash=item2abcfda0a9:g:OKYAAOSwWXFb28Ys:rk:44
f:0
https://www.ebay.com/itm/Mopar-Dodge-8-3-4-Shock-Plates-Hardware-8-1-4-Dana-60-A-B-C-E-Dart-Duster7-Hemi/132934646022?hash=item1ef385b906:g:vukAAOSw9D1cUNn7:sc:USPSPriorityFlatRateBox!73127!US!-1
Can still see the OE part numbers on this used pair.
https://www.ebay.com/itm/Dodge-Plymouth-8-3-4-9-3-4-Dana-Rearend-Leaf-Spring-Shock-Mounting-Plates/253971986168?hash=item3b21e8f6f8:g:FuwAAOSw3ydV1IwL:rk:47f:0
This one has U-bolts (by the way – just look for any ABCE-body U-bolds. ’69 is a good year to look up (they are the same for any pre-ISO clamp system). I highly recommend using NEW U-bolts/nuts (Chrysler calls the U-bolts – Clips, for some reason).
Even if reusing the original ISO-clamp – I still highly recommend using NEW U-bolts/nuts (even if you can get them off, intact).
All FMJ differentials have a hole in mounting perch which is (I believe) 1-7/16” in diameter. A person will need to fill that hole with a piece of metal or something to match the bolt on leaf spring (which is typically 1/2” diameter). Some welding might be needed. I have taken some used washered nuts and placed in the hole that make for great spacers – but I do not recall what they came from. Several options here – but you do need something solid to keep the differential centered on springs – or bad things will happen over time.
If a person is replacing an FMJ differential (either 7¼” or 8¼”) with an ’65-70 8¾” version – then shock plates is usually all you need (it fits like a glove, err, a rather tight glove).
One last thing, the Pre-ISO clamp leaf-spring "centering bolt" is slightly different diameter (the head and nut portion) than the post-ISO clamp centering bolt is - so a person can either drill out the shock plate to the wider ISO centering bolt or replace the bolt with older style. Either method works. If you replace the bolt, get two clamps and tighten the leaf spring - on both sides of the centering bolt. Remove and replace the bolt. Loosen and remove your clamps and that job is done (other than purchasing a new pair of odd shaped bolts). I think drilling out the shock plates might be faster/cheaper - but you won't be able to reuse the plates on something else down the road . . .
The second picture down on this post, the FFI kit. The center lower section of that picture – they include new spring center-bolts as well as washers to fill the differential perch holes. Their kit is expensive ($265 US) – but it has all needed parts to get rid of the ISO clamps.
I do not like working on shocks (front or rear) that use the stud/nut design (like FMJ rear shocks lower attachment) – so using a B-body shock is a plus, to me (easier replacement – down the road).
Don’t even get me going about the front FMJ shocks (with the dual stud/nut design).
I hope this helps
BudW
