Reese hitch on '81 Chev Silverado 4x4 1/2 ton pickup

[Frankenstien]

Just wondering if anyone here has run into my situation fitting (what I believe to be) a Reese OEM receiver-style hitch to my 8' bed fleetside p/u.
I got a good used hitch off a similar 2WD wreck with the same deep-drop bumper as mine.
That bumper uses a couple of 'C' shaped braces which need to be swapped from one side to the other (or removed completely) so that they don't interfere with the hitch installation. Unfortunately for me, when I bought it, I only checked to make sure the pre-existing frame holes matched the hitch bolt pattern without actually getting the 'C' braces cleared out of the way first. Now that they are off, I have run into about 3/8" or more interference from the forward edges of the rear leaf spring mounts when trying to bolt up the front end of the hitch - it is prevented from bolting up flush to the frame.
Reese says my hitch looks like a discontinued "35510, original design". JPG photos attached.
Can anyone confirm this hitch was factory (or dealer) supplied, and, recommend a course of action to modify the hitch, or the spring mounts, or space the hitch away from the frame at all four points, or, look for a different hitch?
Thanks for any help.

[Frankenstien]

As an addendum, I wound up having to grind away four curved recesses in the top of the hitch lateral frame rails to accomodate the rear leaf aft spring mounts; almost a 1/2 inch for the forward edges of the mounts and almost 3/8 of an inch for the rear edges of the mounts ... this on the advice of and by a reputable local RV trailer fabricator. I cleaned up their work to uniform and smooth proportions and then re-painted the entire hitch with several coats of Zero Rust ZER28-62SP black spray paint. I've used Grade 8, 1 1/2" x 1/2"-13NC bolts and nylock nuts with the original galvanized square plate washers (top & bottom) to mount the hitch and torqued all to 64 ft./lbs. Even if some authority was to object to the modification of the hitch in this manner, the finishing work makes it extremely difficult to detect at this point.
BTW - the 'C' shaped braces don't fit worth a **** when swapped L for R, however, as I won't be relying on the step-bumper for anything more than Class I duties, I don't see any harm in leaving them off.