Although not mentioned in the album credits it is quite possible that Duane Allman plays electric slide guitar on "More Than Enough Rain.
Source: Stuart Krause: 'Duane Allman: The Studio Recordings'
published in the November 2005 issue of 'Discoveries':

"In my never-ending search for “lost” Duane Allman recordings, I stumbled upon a copy of First Peace by Bobby Lance. The album was released in 1971 on Cotillion Records, recorded at Muscle Shoals Sound, and touts among its musicians Hinton, Hood, Hawkins, and Beckett with guest appearances by King Curtis and The Sweet Inspirations. One cut, “More than Enough Rain”, stands head and shoulders above the rest, with a blistering electric slide solo that is very much in the style of Duane Allman, quite possibly Duane himself. On the album jacket the slide guitar is credited to Eddie Hinton, but I tracked down Lance in New York City to verify. Lance said he has often been asked this question and has been unable to either confirm or deny the rumors. Lance went down to Criteria to overdub vocals to the rhythm tracks, using the same studio in the morning that The Dominos were using later in the day for Layla. Though Lance had been producing his own album, engineer Dowd offered to remix a couple of songs with which he was not altogether satisfied. According to Lance, Dowd most likely took advantage of Duane's availability and could easily have overdubbed Duane on guitar to create the brilliant interplay between Curtis’ sax solos and the searing slide solos. Lance left Miami after recording his vocal parts and left Dowd in charge of the remix. Lance’s relationship with Atlantic deteriorated soon after. After one more album, Lance left Atlantic and the recording business, disillusioned by the company’s lackluster promotion of his albums due to the preexisting contractual agreements he had with Motown as a songwriter. However, he considers the two Atlantic albums “the best damn albums you’ve never heard” and enthusiastically recalls sharing a hotel, among other things, with Clapton and Allman, late in the summer of 1970."

This track is also released on the 7-CD box set 'Skydog: The Duane Allman Retrospective' (Rounder Records 11661-9137-2, 2013).

From the liner notes:
"It's been debated for some time whether the slide guitar on "More Than Enough Rain" is played by Duane Allman, or by Eddie Hinton, or possibly by both musicians. Unfortunately, documentation does not confirm either scenario. The track has been included because we feel that it is Duane contributing his slide guitar playing. If, by chance, it is not Duane playing, the slide guitar work is certainly influenced by his unique style and technique."

Update September 30, 2013:

Recordings for this album were made on July 20, 1970 at Muscle Shoals Sound Studios in Muscle Shoals, AL and overdubs were recorded in October 1970 at Criteria Studios in Miami, FL.
Recording engineer Ron Albert says he worked on this project and Duane Allman played on it.

(Thanks to Scott Cantor for this information and for contacting Ron Albert)

Update March 22, 2014:

On March 22, 2014 Scott Cantor spoke with David Hood (in the 1960's studio bassist for FAME and also co-founder of the Muscle Shoals Rhythm Section and Muscle Shoals Sound Studio) and Jimmy Johnson (in the 1960's studio guitarist and sound engineer for Rick Hall's FAME Studio in Muscle Shoals, AL. and in 1969 co-founder of the Muscle Shoals Rhythm Section and Muscle Shoals Sound Studio). They believe it is Eddie Hinton or Pete Carr on track 4.

(Thanks to Scott Cantor for this information)

Update February 12, 2015:

From Bill Kopp's liner notes of the 2015 CD release "Bobby Lance: First Peace / Rollin' Man" (Real Gone Music RGM-0332):
".....Famed session man Eddie Hinton handled the lead and slide guitar on all but one cut. And though he's not credited anywhere on the original LP, for First Peace's "More Than Enough Rain," Duane Allman provided some stinging electric slide guitar.
Though it has been the subject of speculation for decades, Lance vigorously confirms Allman's presence on "More Than Enough Rain." The chronology of Allman's involvement goes like this: Bobby had self-produced the sessions for First Peace, but remained dissatisfied with the completed mixes for a few of the album's tracks. Ace producer Tom Dowd offered to remix those tracks, so Lance traveled to Dowd's Miami, FL Criteria Recording Studio (also known as Atlantic Records South), bringing along the multi-track master tapes. Under Dowd's supervision, Lance overdubbed some vocal tracks, and left the tapes with Dowd. Soon thereafter - October '70 - Allman was at Criteria for the sessions that would produce Ronnie Hawkins' self-titled 1970 LP. In a spare moment during those sessions, Dowd asked the guitarist to lay down some licks to improve Lance's track; the resulting mix of "More Than Enough Rain" features musical sparring between King Curtis' horn lines and Duane Allman's impromptu yet fluid slide guitar work (the track also appears on Rounder's 2013 box set, Skydog: the Duane Allman Retrospective)."

Update July 3, 2015:

Recently Don 'dzobo' Sobocinski was in communication with Bill Kopp and asked him whether Eddie Hinton is also on the track.
This is Bill Kopp's response:

"I’m not sure about whether Eddie laid down a part, but it’s certainly possible. Maybe what we’re hearing has both of ‘em. Not the likeliest scenario, but definitely not beyond the realm of possibility."

(Thanks to Don Sobocinski for this information)